Archived 21: National Advisory Committee on Immunization statement: Interim guidance on booster COVID-19 vaccine doses in Canada [2021-10-29]

Published: October 29, 2021

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This is an archived version. Please refer to NACI statements and publications for current COVID-19 vaccine statements.

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Preamble

The National Advisory Committee on Immunization (NACI) is an External Advisory Body that provides the Public Health Agency of Canada (PHAC) with independent, ongoing and timely medical, scientific, and public health advice in response to questions from PHAC relating to immunization.

In addition to burden of disease and vaccine characteristics, PHAC has expanded the mandate of NACI to include the systematic consideration of programmatic factors in developing evidence-based recommendations to facilitate timely decision-making for publicly funded vaccine programs at provincial and territorial levels.

The additional factors to be systematically considered by NACI include: economics, ethics, equity, feasibility, and acceptability. Not all NACI Statements will require in-depth analyses of all programmatic factors. While systematic consideration of programmatic factors will be conducted using evidence-informed tools to identify distinct issues that could impact decision-making for recommendation development, only distinct issues identified as being specific to the vaccine or vaccine-preventable disease will be included.

This statement contains NACI's independent advice and recommendations, which are based upon the best current available scientific knowledge. This document is being disseminated for information purposes. People administering the vaccine should also be aware of the contents of the relevant product monograph. Recommendations for use and other information set out herein may differ from that set out in the product monographs of the Canadian manufacturers of the vaccines. Manufacturer(s) have sought approval of the vaccines and provided evidence as to its safety and efficacy only when it is used in accordance with the product monographs. NACI members and liaison members conduct themselves within the context of PHAC's Policy on Conflict of Interest, including yearly declaration of potential conflict of interest.

Background

NACI's recommendations on booster doses will be based on the decision-making framework outlined in this document, triggered by evidence of the need for (e.g., evidence of decreased vaccine effectiveness against severe illness and/or infection depending on the population) and benefit of (e.g., safety and effectiveness) a booster dose in the Canadian context.

The public health goal of Canada's pandemic response is to minimize serious illness and death while minimizing societal disruption as a result of the COVID-19 pandemic. COVID-19 vaccines have played a vital role in the response and have been shown to be very effective against symptomatic laboratory confirmed SARS-CoV-2 infection, severe disease, hospitalization, and death from COVID-19. Unfortunately, the COVID-19 pandemic is ongoing and continues to cause significant morbidity and mortality, as well as social and economic disruption in Canada and worldwide (including impacts on health system capacity). COVID-19 vaccination with a complete primary series is critical. Fully vaccinated individuals have much lower rates of SARS-CoV-2 hospitalizations, ICU admission and mortality compared to those who are unvaccinated. In addition, those who have been vaccinated are less likely to get infected, and therefore less likely to transmit SARS-CoV-2 infection to others. NACI continues to strongly recommend that all individuals in the authorized age groups should be immunized with a primary series of an authorized COVID-19 vaccine, and preferably with mRNA COVID-19 vaccines (Moderna Spikevax and Pfizer-BioNTech Comirnaty)Footnote1.

To date, COVID-19 vaccines have been shown to maintain high vaccine effectiveness (VE) against serious illness, hospitalization, and death from COVID-19 in most populations. However, evidence is emerging that VE against asymptomatic infection and mild COVID-19 disease may decrease with time, and that currently authorized COVID-19 vaccines may be less effective against the highly transmissible Delta variant (B.1.617.2), which could contribute to increased transmission of infection. Therefore, an additional or booster dose may be needed to obtain more durable protection in some populations.

Evidence from clinical trials suggests that booster doses of mRNA vaccines given six months after the primary series elicited a robust immune response against the wild type strain and variants of Concern (VoC), with titres often higher after the booster dose than after the primary series. Real-world data from Israel suggest that a booster dose provides good short-term effectiveness against SARS-CoV-2 infection and has a safety profile comparable to that observed after the second dose of the vaccine.

The intent of a booster dose is to restore protection that may have decreased over time to a level that is no longer deemed sufficient in individuals who initially responded adequately to a complete primary vaccine series. This is distinguished from the intent of an additional dose which might be added to the standard primary vaccine series with the aim of enhancing the immune response and establishing an adequate level of protection. For example, evidence suggests that compared to the general population, individuals who are moderately to severely immunocompromised have lower immune responses to COVID-19 vaccines. Therefore, NACI has recommended that moderately to severely immunocompromised individuals in the authorized age groups should be immunized with a primary series of three doses of an authorized mRNA vaccine.

Historically in other vaccine programs, it can take years of post-market use to determine the optimal intervals and dose number needed for a complete primary series to sustain long-term protection. At present, there is scientific debate about whether a third dose for COVID-19 vaccines truly constitutes a booster dose in the traditional sense. NACI continues to monitor the emerging scientific data on how best to use these vaccines, and will study the important differences between a primary series (to establish strong immune memory), versus a booster (to stimulate the memory response once protection has truly waned). Over time, it may be learned that a short 2-dose primary series, with a booster at least 6 months after the second dose, can in fact be adjusted to achieve durable protection with a more streamlined primary series. For example, NACI has already highlighted benefit in terms of longer-term protection when the second dose is provided at least 8 weeks after the first dose. In this guidance document, additional doses of COVID-19 vaccines after the authorized series are being described as booster doses but it should be acknowledged that over time, what defines an optimal primary series could also evolve and be refined.

NACI's recommendations on booster doses are occurring in the context of the World Health Organization's (WHO's) call for global vaccine equity, and take into consideration conclusions in its Interim statement on booster doses for COVID-19 vaccination including the call for evidence-based decisions: "Introducing booster doses should be firmly evidence-driven and targeted to the population groups in greatest need. The rationale for implementing booster doses should be guided by evidence on waning vaccine effectiveness, in particular a decline in protection against severe disease in the general population and in high-risk populations, or due to a circulating VoC. To date, the evidence remains limited and still inconclusive on any widespread need for booster doses following a primary vaccination series. The focus remains on urgently increasing global vaccination coverage with the primary seriesFootnote2." NACI's recommendations on booster doses in those who have completed a primary series will be triggered by evidence on the need for a booster dose (in key populations at increased risk or in the general population), as well as the benefit of a booster dose.

Internationally, several countries, including the United StatesFootnote3, the United KingdomFootnote4, FranceFootnote5, and GermanyFootnote6, have recently recommended booster doses of COVID-19 vaccines at least 6 months following a primary vaccine series for certain high-risk groups, such as older adults, long-term care residents, and healthcare workers. Israel initially recommended a booster dose in adults 60 years of age and older and subsequently recommended a booster dose for the general population 12 years of age and over, at least 5 months following the primary series administered at the authorized dosage intervalFootnote7.

Countries that have rolled out primary series of COVID-19 vaccines using different vaccines and different intervals between doses of vaccines are experiencing different levels of protection over time, which is to be expected. NACI's recommendations for booster doses will differ from recommendations in other countries because of differences in a number of contextual factors including:

NACI reviewed available evidence on the factors presented in Table 1 in the context of the current Canadian epidemiology, vaccine programs, and vaccine schedules. Over 80% of Canadians aged 12 years and older have completed a primary COVID-19 vaccine series. Most are at a lower risk of declining protection due to receipt of mRNA vaccines (following NACI's preferential recommendation for mRNA vaccinesFootnote1) or a combination of vaccine products in some instances (following NACI's recommendation on the interchangeability of authorized COVID-19 vaccinesFootnote8), and at intervals longer than the manufacturer authorized intervals (following NACI's recommendation for extended intervalsFootnote9). Furthermore, the Moderna Spikevax vaccine, authorized for use in Canada, appears to offer more durable protection against severe disease and asymptomatic infectionFootnote10. There is no evidence of decreasing protection over time against severe disease in the general Canadian population who have been vaccinated against COVID-19 disease. To date, Health Canada has not authorized booster doses of COVID-19 vaccines. NACI will continue to closely monitor the evidence and encourages a coordinated evidence-informed national approach.

On September 28, 2021, NACI recommended that a booster dose of an authorized mRNA vaccine should be offered to all long-term care residents and seniors living in other congregate settings who have received a primary COVID-19 vaccine series (with the primary series being a homologous or heterologous schedule using mRNA and/or viral vector vaccines) at an interval of at least 6 months after the primary series has been completedFootnote11. This population was also initially prioritized as a key population to receive initial doses of a primary series of COVID-19 vaccines based on evidence of an increased risk of severe illness and death and increased risk of exposure to SARS-CoV-2. The recommendation for a booster dose was triggered by increases in COVID-19 cases and outbreaks in long-term care homes with signs emerging that protection from vaccination might not persist as long in this population compared to other populations in Canada. In addition, long-term care residents are at high risk of exposure to SARS-CoV-2 due to their congregate living environment and at high risk of severe outcomes due to age and underlying health status. Longer time since last dose and shorter intervals between doses in the primary series, as well as older age/immunosenescence, also contribute to waning vaccine protection against infection and severe outcomes in this population. Assessment of the need for and benefit of a booster dose in other populations based on the criteria in Table 1 in the Canadian context and NACI's decision-making framework inform and guide NACI's recommendations herein subsequent to the Rapid response: Booster doses in long-term care residents and seniors living in other congregate care settings.

Guidance objective

The objective of this advisory committee statement is to provide evidence-informed guidance on the equitable, ethical, and effective use of additional doses of authorized COVID-19 vaccines in the Canadian context based on the need for, and benefit of, booster doses to minimize serious illness and deaths while minimizing societal disruption as a result of COVID-19.

Methods

The evidence pertaining to COVID-19 and COVID-19 vaccines is rapidly evolving. NACI reviewed the decision-making framework and evidence on the need for and benefit of additional doses of COVID-19 vaccines in various populations on September 7, 14, 27, October 12 and 15, 2021. NACI consulted with the Public Health Ethics Consultative Group (PHECG) on the ethical implications of booster dose recommendations in various populations on September 2 and 21, 2021. Following a comprehensive review of available evidence and consultations with the provinces and territories through the Canadian Immunization Committee (CIC) and the Chief Medical Officers of Health (CMOH), NACI made and approved these recommendations on October 22, 2021.

NACI's decision-making framework on booster doses was modified from NACI's original prioritization framework of key populations for COVID-19 vaccination. The evidence supporting the development of the original framework is summarized in NACI's previously published guidance:

  1. Preliminary guidance on key populations for early COVID-19 immunization (November 2020)
  2. Guidance on the prioritization of initial doses of COVID-19 vaccines (December 2020)
  3. Guidance on the prioritization of key populations for COVID-19 immunization (February 2021)

To guide ethical decisions that are based on evidence and on clear, transparent criteria, NACI developed a decision-making framework for booster doses modified from its original evidence-informed prioritization framework for COVID-19 vaccinationFootnote12. NACI's recommendations on booster doses will be based on this decision-making framework, triggered by evidence of the need for, and benefit of, a booster dose in the Canadian context (Table 1).

Key populations prioritized for a primary series of COVID-19 vaccination in NACI's original framework were based on evidence of increased risk of severe illness and death from COVID-19 and increased risk of exposure to SARS-CoV-2, summarized in NACI's 2020 guidanceFootnote12Footnote13Footnote14Footnote15 Footnote16Footnote17. NACI's decision-making framework on booster doses also considered populations with emerging evidence suggesting decreased protection from the primary series (e.g., vaccination with only viral vector vaccines, a longer time since completion of the primary series, shorter interval between doses of the primary series). NACI's recommendations are also guided by ethics and rooted in the foundational elements of equity, feasibility and acceptability

Table 1. Underlying factors for consideration based on evolving evidence to determine the need for and benefit of a booster dose of COVID-19 vaccine in various populations
Underlying factors for consideration Evidence reviewed to determine the need for and benefit of a booster dose of COVID-19 vaccine
Risk benefit analysis
  • Risk of severe illness and death
  • Risk of exposure (including ability to physically distance and access to infection prevention and control
  • measures and healthcare)
  • Risk of transmission to vulnerable populations
  • Risk of societal disruption
Vaccine characteristics in different groups against wild-type and VoC
  • Duration of protection
  • Immunogenicity
  • Efficacy/effectiveness
  • Safety and reactogenicity of boosters
  • Effect of vaccine in preventing transmission
Vaccine supply/types/intervals
  • Number and type of available vaccines
  • Initial vaccination series (type, interval between doses, time since initial series)
COVID-19 epidemic conditions
  • Circulation of SARS-CoV-2 wild-type and VoC
  • Breakthrough cases, outbreaks
  • Case rates and implications for health system capacity

NACI recommendations on the use of COVID-19 vaccines are available.

Data on COVID-19 vaccination coverage and doses administered in various key populations in jurisdictions across Canada is available.

Further information on NACI's process and procedures is available elsewhereFootnote18Footnote19.

Summary of evidence

Vaccine principles for booster doses

The immune responses to a vaccine are determined by a number of factors including vaccine type, interval between doses in the primary series, time since completion of the primary series, and underlying health status and age.

Higher antibody titres occur with the Moderna vaccine compared to the Pfizer-BioNTech vaccine and both have a higher titre than the viral vector vaccinesFootnote20. A longer interval between the first and second doses also results in higher titresFootnote21Footnote22. Although correlates of protection against SARS-CoV-2 have not yet been clearly defined, a higher antibody titre appears to be associated with longer duration of protection against symptomatic infection, including against VoC.

While there are various studies showing decreasing levels of circulating neutralizing antibodies as well as binding antibodies over time, studies also show that the mRNA vaccines elicit a memory B and T cell responseFootnote23Footnote24. Even if circulating antibodies decrease, future exposure to SARS-CoV-2 is expected to drive a 'recall' response and long-lived memory T and B cells will help produce new antibodies. Therefore, even if a vaccinated individual is infected with SARS-CoV-2, vaccine-induced immunity through immune memory is expected to help to prevent progression to severe disease in most individuals, although the duration of immune memory is not known at this time.

For more information on vaccine principles, please consult the chapter on basic immunology and vaccinology in the Canadian Immunization Guide.

Recent COVID-19 epidemiological trends

There is currently a resurgence of COVID-19 cases in regions of Canada fuelled by the highly transmissible Delta variant. Outbreaks continue to occur in multiple settings, including long-term care homes and retirement residences, industrial settings, school and daycare settings, as well as other settings that are enclosed and crowded, and can be a significant source of spread of SARS-CoV-2 infection. School and daycare settings have experienced an increasing number of outbreaks since mid-AugustFootnote25 due in part to a large proportion of ineligible and unvaccinated population (children under 12). In early August, the rate of active cases started rising in First Nations communities for the first time since mid-January 2021, and was 4.2 times higher than the rate in the general population as of October 12Footnote26. As such, this NACI guidance is provided in the midst of the fourth COVID-19 pandemic wave driven by the Delta variant.

Canadian surveillance data up to October 2, 2021 shows that rates of new SARS-CoV-2 infection are highest among persons who are unvaccinated and lowest in persons who are fully vaccinated. Unvaccinated persons have also had much higher rates of hospitalizations, ICU admission and deaths compared to those fully vaccinated. Compared to those who are fully vaccinated, the rate of SARS-CoV-2 infection in unvaccinated persons was 8 times higher and the rate of COVID-19 related hospitalization in unvaccinated persons was 25 times higher, on average, for each week during the period of September 5 to October 2, 2021. While the incidence rate of infection is much lower in fully vaccinated people, it increased slightly across all age groups since mid-July, but has declined as of the week of September 26 - October 2, 2021 for all age groups.

Compared to fully vaccinated younger age groups, fully vaccinated cases 80 years of age and over have the highest rates of hospitalizations and deaths, followed by those aged 70 to 79 years. Among the fully vaccinated, these older age groups have the highest proportion of cases who are hospitalized and who have died from COVID-19. The weekly proportions of fully vaccinated cases who are hospitalized or who died has remained relatively low and stable since mid-July and the case fatality has decreased more recently in the older age groups, indicating that fully vaccinated people who become infected do not appear to be getting more severely ill over time.

Duration of COVID-19 vaccine protection against infection

Emerging evidence suggests a decrease in COVID-19 vaccine protection against SARS-CoV-2 infection over time following completion of the primary series. However, it can be challenging to distinguish potential signals of waning from increasing case numbers driven by community spread during the fourth wave of the pandemic and the rise of the Delta variant. Evidence on increasing incidence of infection in vaccinated individuals coincides with periods when the Delta variant predominated, and estimates of lower VE may be a reflection of decreased effectiveness against the Delta variant rather than waning in COVID-19 vaccine protection. Further, increasing incidence in vaccinated individuals may also be observed in areas with lower vaccine coverage as a result of overall higher community rates driven by SARS-CoV-2 infection in the unvaccinated population. Continued research evaluating VE is needed to accurately determine trends in protection over time, as well as to learn more about the effects on transmission and the magnitude, if any, of potential decrease in protection. Immunogenicity data alone is insufficient to assess waning of protection against disease, and may not be indicative of protection against severe outcomes. To date, protection against severe COVID-19 outcomes, such as hospitalization and death, consistently appear to be more durable than protection against infection. There are some data that suggest decreases in protection may be greater in older age groups and in individuals with clinical risk factors for more severe outcomesFootnote27Footnote28.

A recent rapid reviewFootnote29 on vaccine efficacy/effectiveness over time in COVID-19 vaccinated individuals identified seven studies that examined vaccine efficacy/effectiveness longitudinally over a period of 4 months or longer and provided both baseline and follow-up data. Studies that reported on confirmed infectionFootnote30Footnote31Footnote32 as an outcome generally indicated a decrease in VE against SARS-CoV-2 infection at 4 and 6 months after primary series completion compared to 7 to 14 days after primary series completion. Trends were similar for studies reporting on symptomatic infectionFootnote27Footnote30Footnote33Footnote34. In contrast, the studies that reported on COVID-19 related hospitalizationFootnote27Footnote31Footnote32Footnote35 and deathsFootnote27Footnote30Footnote32 indicated that VE against severe COVID-19 outcomes remained stable over time thus far. These patterns were generally similar across vaccine products and in individuals over 60 years old. However, evidence was limited by the small number of heterogeneous studies, which were observational in design.

Studies on duration of protection have typically examined protection after a manufacturer-recommended dosing interval of 3 or 4 weeks between first and second doses for mRNA vaccines. It is currently uncertain how a longer interval between first and second vaccine doses in a primary series might affect the duration of protection. Provincial data from British Columbia and Quebec found that shorter intervals between doses in a primary series result in lower VE against SARS-CoV-2 infection and COVID-19 related hospitalizations compared to extended intervals. Further, emerging evidence suggests that shorter intervals between doses may be associated with lower VE against infection over timeFootnote27Footnote28. Evidence to date suggests that delaying the second dose by several weeks leads to higher antibody titres and greater VE of the seriesFootnote22Footnote36Footnote37 which is likely to result in a more durable immune response and longer protection over time.

It is currently unclear to what extent the duration of protection may vary by vaccine product. In general, VE against symptomatic SARS-CoV-2 infection and severe COVID-19 outcomes has consistently been somewhat lower with viral vector vaccines compared to mRNA vaccinesFootnote38. Emerging data on effectiveness suggests that vaccine protection against infection and symptomatic disease decreases more quickly with viral vector vaccines in comparison to mRNA vaccines, whereas the difference is less pronounced for severe diseaseFootnote27Footnote28. Limited real-world data from Canada and the United States suggests that protection from Moderna Spikevax may be more durable compared to Pfizer-BioNTech ComirnatyFootnote28Footnote39, but more research is required.

There is limited evidence on duration of protection following a mixed COVID-19 vaccination schedule. Data from two studies indicate that VE for those who received a mixed schedule of AstraZeneca Vaxzevria/COVISHIELD followed by an mRNA vaccine is similar compared to those who received a complete series of mRNA vaccinesFootnote28Footnote40.

Despite some evidence of increasing risk of breakthrough infection over time, those vaccinated against COVID-19 with a two-dose series continue to demonstrate significantly lower odds of SARS-CoV-2 infection compared to unvaccinated individuals and, when infections occur, symptoms tend to be milder in vaccinated casesFootnote41. VE against severe COVID-19 outcomes with all vaccine types remains high, even in the context of the Delta variant. Breakthrough infections in vaccinated persons could contribute to ongoing transmission of SARS-CoV-2. Early evidence from when the Alpha variant predominated suggested that vaccinated individuals who became infected were less infectiousFootnote42. The evidence on transmission with the Delta variant is less clear, with some studies suggesting the differences in viral load between vaccinated and unvaccinated persons who become infected may be less compared to when the Alpha variant was predominantFootnote42Footnote43Footnote44Footnote45Footnote46.

Immunogenicity, safety, and effectiveness of COVID-19 vaccine booster doses

Ongoing manufacturer-sponsored trials on mRNA vaccines have reported higher titres following the third doses compared to those after the initial seriesFootnote47Footnote48 of two doses (administered at manufacturer authorized intervals), suggesting that the higher titres produced by a booster dose may lead to longer lasting protection than the primary series administered at manufacturer authorized intervals. Early results also show a favourable reactogenicity profile for booster mRNA vaccine doses, similar to that of the second dose in the primary seriesFootnote47Footnote48. Evidence from these trial data is limited by small sample size (less than 350 participants in each published manufacturer-sponsored trialFootnote49Footnote50) and short duration of follow-up of study populations. Pfizer-BioNTech Comirnaty and Moderna Spikevax have filed submissions for booster doses ≥ 6 months after the primary series to Health Canada for regulatory approval as of October 1 and 5, 2021 respectively. The regulatory submission for a Moderna Spikevax booster dose is for half the current dosage of Moderna Spikevax primary series dose (i.e., a 50 mcg booster dose vs. 100 mcg full dose). The regulatory submission for a Pfizer-BioNTech Comirnaty booster dose is the same as the current dosage of the primary series dose for this vaccine (i.e., a 30 mcg booster dose).

Emerging real-world data from Israel's booster dose program with Pfizer-BioNTech Comirnaty indicates that a third dose (after a primary series using the manufacturer authorized interval of 21 days between doses) resulted in improved short-term vaccine effectiveness against infection and severe illnessFootnote51. In one Israeli study of individuals ≥ 60 years of age, a booster dose of Pfizer-BioNTech Comirnaty at least 5 months after the primary series decreased the relative risk of confirmed SARS-CoV-2 infection by 11.3-fold and of severe illness by 19.5-fold at 12 or more days from the booster dose, compared to those with two dosesFootnote52. An extension of this analysisFootnote53 found that, compared to a two-dose series, a booster dose resulted in about a 10-fold reduction in confirmed infection rates in persons ≥ 16 years of age. In another Israeli study of persons ≥ 40 years of age, those who received a third dose had a 70 to 84% reduction in the odds of testing positive for SARS-CoV-2 infection 14 to 20 days after receiving the booster compared to people who received two doses of Pfizer-BioNTech ComirnatyFootnote54. There are no data currently on the long-term effectiveness of booster doses so it remains unknown at this time how long benefit might last. The effect of booster doses on transmission is unknown.

Studies evaluating boosters following different primary series vaccine schedules are ongoingFootnote55Footnote56. Unpublished data from the Cov-Boost trial presented to the United Kingdom's Joint Committee on Vaccination and Immunisation (JCVI) suggest that mRNA booster doses are generally well tolerated and provide a strong booster effect regardless of the vaccine used in the primary seriesFootnote4. Similarly, recent data from the US National Institutes of Health "Mix and Match" trial indicates that heterologous booster doses given at least 12 weeks following completion of the primary series of mRNA vaccines or Janssen COVID-19 vaccine were well-tolerated and immunogenic. Additionally, those who received an mRNA booster following a dose of Janssen COVID-19 vaccine had higher antibody titres compared to those who received a second dose of Janssen as a boosterFootnote57.

The safety and effectiveness of a third dose in persons who had a previous SARS-CoV-2 infection is currently unknown.

Rare cases of myocarditis and pericarditis following vaccination with COVID-19 mRNA vaccines have been reported, more frequently after the second dose compared to the first dose, and more commonly in younger males and adolescents. Canadian data also suggest that myocarditis/pericarditis occur more frequently after Moderna Spikevax compared to Pfizer Comirnaty COVID-19 vaccines. The rate of myocarditis and pericarditis following a booster dose of a COVID-19 mRNA vaccine is currently unknown. Initial surveillance data from Israel up to October 10, 2021 has reported 17 cases of myocarditis or peri-myocarditis out of approximately 3.7 million booster doses of Pfizer-BioNTech Comirnaty administeredFootnote51. In Israel, this rate is lower than observed after the second dose, but higher than observed after the first dose. Data collection is ongoing. NACI will continue to monitor the evidence and update recommendations as needed.

Optimal primary series to booster dose interval

There are currently limited data to determine the optimal interval between the completion of the primary series and administration of the booster dose. Most studies on mRNA COVID-19 vaccine booster doses have used an interval of 6 months or more following the completion of the primary series, although some have used an interval as short as 3 monthsFootnote55Footnote56. Submissions filed with regulatory authorities in the US, EU and Canada are for 6 months or more following the second dose, which was the interval used in booster doses trials for Pfizer-BioNTech Comirnaty and Moderna SpikevaxFootnote49Footnote50. However, it is currently unknown at what interval a maximum boosting effect is achieved. For older adults who may have a decrease in protection over time, delaying the booster dose will increase the period during which individuals may have reduced protection against SARS-CoV-2 infection, although to date protection against severe outcomes has been shown to be more durable than protection against infection.

Summary of primary COVID-19 vaccine series that have been used in Canada to date

Vaccine types received in the primary series in Canada

As of October 9, 2021, 82% of eligible Canadians have been fully vaccinated with a COVID-19 vaccine, while 87% have received at least one dose. Of those fully vaccinated, the majority received a complete two-dose series of mRNA vaccines. A small percentage received a complete series with a viral vector vaccine. At least 469,371 Canadians have received a viral vector vaccine primary series and 1,395,324 Canadians have received a heterologous primary series containing both a viral vector vaccine and an mRNA vaccine. Almost all viral vector primary series were with AstraZeneca Vaxzevria/COVISHIELD vaccines. Data on vaccination coverage by vaccine product was missing for two provinces.

Refer to COVID-19 vaccination in Canada for the most current information on vaccination coverage.

Intervals between doses in the primary series of COVID-19 vaccines in Canada

Shorter intervals between doses results in lower antibody titres which may wane to below protective levels over time. While individuals who received their second dose in the primary COVID-19 vaccine series at a shorter interval from the first dose were well protected in the short-term, they may have produced lower antibody levels, which may decrease over time compared with those who had a longer interval between doses.

Intervals between the first and second doses of a two-dose primary series of COVID-19 vaccines varied across Canada as vaccine supply and evidence evolved. Groups prioritized for vaccination early in the vaccine roll-outFootnote12 often received their vaccines using the manufacturers' recommended interval of 21 days for Pfizer-BioNTech Comirnaty and 28 days (or as short as 21 days) for Moderna Spikevax. Subsequently, intervals between doses were extended up to 16 weeks to optimize early vaccine rollout and population protection in Canada in the context of limited vaccine supplyFootnote9. As vaccine supply was no longer limited, and in the context of the increasing prevalence of the Delta variant, jurisdictions accelerated second doses with shorter intervals. Aggregated vaccination coverage data obtained from provincial and territorial vaccination registries up to August 14, 2021, showed that an interval of 7-11 weeks between first and second doses was the most common dosing interval across all vaccine products. Dosing intervals varied widely by jurisdiction and age group. Most notably, 66% of vaccinated adults aged 80 years old and older had an interval of 12 weeks or more between first and second dose, while 9% had an interval of 28 days or less. Data on vaccination coverage by dosing interval was missing for one province.

There is evidence that the Moderna Spikevax vaccine remains efficacious against severe disease and asymptomatic infection at more than 5 months when given at the authorized interval of 28 days between dosesFootnote10. There is evidence that while the Pfizer-BioNTech Comirnaty vaccine prevents COVID-19 for up to 6 months, there is a gradual decline in efficacy when given at the authorized interval of 21 days between dosesFootnote34. Though limited data suggests that protection from Moderna Spikevax may be more durable compared to Pfizer-BioNTech ComirnatyFootnote28Footnote39, more research is required.

Time since completion of primary COVID-19 vaccine series in Canada

As noted above, protection against infection may decrease with time since completion of the second dose of vaccine. Key populations at highest risk of severe illness due to COVID-19 and/or highest risk of exposure to SARS-CoV-2 (e.g., residents and staff of congregate living settings that provide care for seniors, older adults, frontline healthcare workers, adults in or from Indigenous communities) were prioritized to receive COVID-19 vaccines earlier than others when initial vaccine supply was limitedFootnote12. Therefore, many in these populations would have completed their primary series longer than 6-8 months ago. A number of these key populations received their second doses between January and April 2021. The vast majority of Canadians who are fully vaccinated completed their primary series in June or July 2021 (84%). Only 4% received their second doses between January and April 2021. Data on vaccination coverage by time since last dose was missing for one province.

Ethics, equity, feasibility and acceptability considerations

Ethics

Advice provided to NACI by the PHAC Public Health Ethics Consultative Group (PHECG) on the ethical implications of booster dose recommendations included the followingFootnote58Footnote59:

Equity

Global equity
Domestic equity

Feasibility

Acceptability

Refer to NACI's previous guidance for a comprehensive overview of the ethical, equity, feasibility and acceptability considerations for prioritizing key populations for COVID-19 vaccination Footnote12Footnote13Footnote16Footnote17.

Recommendations

Please see Table 2 for an explanation of strong vs discretionary NACI recommendations.

NACI strongly reiterates its previous evidence-informed recommendations for the primary series of COVID-19 vaccines in all authorized age groups:

1. NACI preferentially recommendsFootnote1 that a complete series with an mRNA COVID-19 vaccine should be offered to individuals in the authorized age group without contraindications to the vaccine. (Strong NACI Recommendation)

Additional details are available in the NACI statement on Recommendations on the use of COVID-19 vaccines.

2. NACI recommendsFootnote65 that moderately to severely immunocompromised individualsEndnote i in the authorized age groups should be immunized with a primary series of three doses of an authorized mRNA vaccine. For those who have previously received a 1- or 2-dose complete primary COVID-19 vaccine series (with a homologous or heterologous schedule using mRNA or viral vector vaccines), NACI recommends that an additional dose of an authorized mRNA COVID-19 vaccine should be offered. (Strong NACI Recommendation)

Additional details are available in the NACI rapid response: Additional dose of COVID-19 vaccine in immunocompromised individuals following 1- or 2- dose primary series.

NACI's evidence-informed recommendations for booster doses of COVID-19 vaccines:

NACI recognizes that epidemiological and logistical/operational contexts, as well as impacts on health system capacity, vary between provinces and territories across Canada. NACI encourages jurisdictions to align with these recommendations as much as possible to ensure the equitable, ethical and effective use of booster doses of COVID-19 vaccines in Canada, maintaining vaccine acceptance and confidence, while considering their local contexts.

NACI also acknowledges that the epidemiology of COVID-19 (including the impact of SARS-CoV-2 variants of concern) and the evidence on booster doses of COVID-19 vaccines are rapidly evolving, and will continue to monitor the evidence in the Canadian context and provide additional recommendations and updates subsequent to this interim statement as data emerge.

Following an evaluation of the need for, and benefit of, additional doses of COVID-19 vaccines based on evolving evidence on the criteria outlined in Table 1, as well as the systematic assessment of ethics, equity, feasibility and acceptability considerations with the EEFA frameworkFootnote19, NACI makes the following evidence-informed recommendations on booster doses of authorized COVID-19 vaccines in the context of ongoing risk of severe illness from COVID-19 and exposure to SARS-CoV-2 and VoCs in Canada:

For key populations at highest risk of severe illness from COVID-19 and highest risk of waning protection:

3. NACI recommends that a booster dose of an authorized mRNA COVID-19 vaccineFootnote* should be offered ≥6 months after completion of a primary COVID-19 vaccine series (where the primary series consisted of a homologous or heterologous schedule using mRNA or viral vector vaccines) to individuals in the following key populations:

(Strong NACI Recommendation)

For key populations at increased risk of severe illness from COVID-19 and increased risk of waning and/or lower protection:

4. NACI recommends that a booster dose of an authorized mRNA COVID-19 vaccineFootnote* may be offered ≥6 months after completion of a primary COVID-19 vaccine series to individuals in the following key populations:

(Discretionary NACI Recommendation)

For key populations who may be at increased risk of severe illness from COVID-19 (due to intersecting social and health risk factorsFootnote13) and waning protection (due to increased time since completion of the primary COVID-19 vaccine series after a shorter interval between doses) where infection can have disproportionate consequencesFootnote12:

5. NACI recommends that a booster dose of an authorized mRNA COVID-19 vaccineFootnote* may be offered ≥6 months after completion of a primary COVID-19 vaccine series (where the primary series consisted of a homologous or heterologous schedule using mRNA or viral vector vaccines) to individuals in the following key population:

(Discretionary NACI Recommendation)

For key populations who are essential for maintaining health system capacity and who may be at increased risk of waning protection (due to increased time since completion of the primary COVID-19 vaccine series after a shorter interval between doses) and who could pose increased risk of transmission to vulnerable populations:

6. NACI recommends that a booster dose of an authorized mRNA COVID-19 vaccineFootnote* may be offered ≥6 months after completion of a primary COVID-19 vaccine series (where the primary series consisted of a homologous or heterologous schedule using mRNA or viral vector vaccines) to individuals in the following key population:

(Discretionary NACI Recommendation)

For other populations not included in the above recommendations for a booster dose, NACI will continue to closely monitor the evidence and will make additional recommendations if there is evidence of the need for, and benefit of, a booster dose. This includes monitoring the specific evidence for:

Footnote 1

Either Moderna Spikevax or Pfizer-BioNTech Comirnaty vaccines may be used as a booster dose (regardless of which COVID-19 vaccine was used in the primary series). As previously recommended, adults living in long-term care homes for seniors or other congregate living settings that provide care for seniors are recommended to receive the full dose (100 mcg) if being offered Moderna Spikevax. For other adults recommended to receive a booster dose, the full dose (100 mcg) is recommended for adults 70 years of age or older, if offering Moderna Spikevax, while a half dose (50 mcg) is recommended for those less than 70 years of age. If offering Pfizer-BioNTech Comirnaty, the full dose (30 mcg) is recommended.

Return to footnote * referrer

Individuals who had a severe immediate allergic reaction (e.g., anaphylaxis) to a previous mRNA vaccine or who have a severe immediate allergic reaction (e.g., anaphylaxis) to a component of the mRNA vaccine should consult with an allergist or other appropriate physician as vaccination with an mRNA has been safely performed in these populations. Additional guidance for individuals with myocarditis/pericarditis after a previous dose of an mRNA vaccine is under consideration and will be forthcoming.

Summary of evidence and rationale

Key populations included in this initial guidance on booster doses of COVID-19 vaccine

Long-term care residents and seniors living in other congregate settings

Older age

Recipients of only viral vector vaccines

Adults in or from First Nations, Inuit and Métis communities

Frontline healthcare workers

NACI is continuing to monitor the evidence related to waning immunity in various populations and the evidence on immunogenicity, safety and effectiveness of booster doses (including those who have been previously infected with SARS-CoV-2 and have received a complete primary vaccine series with authorized COVID-19 vaccines). NACI will update guidance as required.

Refer to NACI's Recommendations on the use of COVID-19 vaccines for further information on COVID-19 vaccines.

Refer to NACI's Guidance on the prioritization of key populations for COVID-19 immunization for further information on NACI's initial framework and foundational elements guiding ethical decision-making.

Table 2. Strength of NACI recommendations
Strength of NACI recommendation based on factors not isolated to strength of evidence(e.g., public health need) Strong Discretionary
Wording "should/should not be offered" "may/may not be offered"
Rationale Known/anticipated advantages outweigh known/anticipated disadvantages ("should"), OR Known/Anticipated disadvantages outweigh known/anticipated advantages ("should not") Known/anticipated advantages are closely balanced with known/anticipated disadvantages, OR uncertainty in the evidence of advantages and disadvantages exists
Implication A strong recommendation applies to most populations/individuals and should be followed unless a clear and compelling rationale for an alternative approach is present. A discretionary recommendation may/may not be offered for some populations/individuals in some circumstances. Alternative approaches may be reasonable.

Research priorities

  1. What is the efficacy, effectiveness, immunogenicity and safety of booster dose COVID-19 vaccine individuals who have had a previous laboratory-confirmed SARS-CoV-2 infection?
  2. What is the effect of booster doses of COVID-19 vaccines on transmission of infection at a population level? How long do any beneficial effects on transmission last?
  3. Is a booster dose required after a 3-dose primary series of COVID-19 vaccines in those who are moderately to severely immunocompromised?
  4. What is the optimal product (including the booster vaccine in relation to the product(s) received for the primary series), booster vaccine dose, interval between doses in the primary series, interval between the primary series and additional/booster dose, and potential need for (and frequency of) future booster doses in groups at high risk for severe COVID-19 outcomes and in the general population to ensure protection against SARS-CoV-2?
  5. What is the optimal timing and trigger for booster doses? What are the risks associated with providing a booster dose earlier than necessary?
  6. Will special adverse events that have been associated with the primary series (e.g., myocarditis/pericarditis) also be associated with additional/booster doses? Will any new or previously unrecognized adverse event occur with booster doses?
  7. What is the efficacy, effectiveness, immunogenicity, and safety of booster doses of COVID-19 vaccine following a complete series across diverse population groups (e.g., adults of advanced age, those with high-risk medical conditions including autoimmune conditions and transplant recipients, individuals with social or occupational vulnerabilities, individuals who are pregnant or breastfeeding, adolescents, frailty)?

Acknowledgments

This statement was prepared by: SJ Ismail, K Farrah, E Wong, B Warshawsky, J Montroy, R Pless, J Zafack, R Krishnan, SH Lim, M Tunis, K Young, R Harrison and S Deeks on behalf of NACI.

NACI gratefully acknowledges the contribution of: K Ramotar, N St-Pierre, C Mauviel, J Rotondo, N Gilbert, N Lapczak, M Salvadori, K Rutledge-Taylor, the Public Health Ethics Consultative Group (PHECG), and the NACI Secretariat.

NACI members: S Deeks (Chair), R Harrison (Vice-Chair), J Bettinger, N Brousseau, P De Wals, E Dubé, V Dubey, K Hildebrand, K Klein, J Papenburg, A Pham-Huy, C Rotstein, B Sander, S Smith, and S Wilson.

Liaison representatives: LM Bucci (Canadian Public Health Association), E Castillo (Society of Obstetricians and Gynaecologists of Canada), A Cohn (Centers for Disease Control and Prevention, United States), L Dupuis (Canadian Nurses Association), J Emili (College of Family Physicians of Canada), D Fell (Canadian Association for Immunization Research and Evaluation), M Lavoie (Council of Chief Medical Officers of Health), D Moore (Canadian Paediatric Society), M Naus (Canadian Immunization Committee), P Emberley (Canadian Pharmacists Association), L Bill (Canadian Indigenous Nurses Association), and S Funnel (Indigenous Physicians Association of Canada).

Ex-officio representatives: V Beswick-Escanlar (National Defence and the Canadian Armed Forces), E Henry (Centre for Immunization and Respiratory Infectious Diseases (CIRID), PHAC), M Lacroix (Public Health Ethics Consultative Group, PHAC), C Lourenco (Biologic and Radiopharmaceutical Drugs Directorate, Health Canada), S Ogunnaike-Cooke (CIRID, PHAC), K Robinson (Marketed Health Products Directorate, HC), G Poliquin (National Microbiology Laboratory, PHAC), and T Wong (First Nations and Inuit Health Branch, Indigenous Services Canada).

NACI High Consequence Infectious Disease Working Group

Members: R Harrison (Chair), Y-G Bui, S Deeks, K Dooling, K Hildebrand, M Miller, M Murti, J Papenburg, R Pless, S Ramanathan, N Stall, and S Vaughan.

PHAC participants: N Abraham, L Coward, N Forbes, C Jensen, A Killikelly, R Krishnan, J Montroy, A Nam, M Patel, M Salvadori, A Sinilaite, R Stirling, E Tice, B Warshawsky, R Ximenes MW Yeung, and J Zafack.

References

Footnote 1

Advisory Committee Statement (ACS): National Advisory Committee on Immunization (NACI). Recommendations on the use of COVID-19 vaccines [Internet]. Ottawa (ON): Public Health Agency of Canada; 2021 Sep 28 [cited 2021 Oct 16]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/recommendations-use-covid-19-vaccines.html.

Return to footnote 1 referrer

Footnote 2

Interim statement on booster doses for COVID-19 vaccination [Internet]. Geneva: WHO; 2021 Oct 4 [cited 2021 Oct 16]. Available from: https://www.who.int/news/item/04-10-2021-interim-statement-on-booster-doses-for-covid-19-vaccination.

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Footnote 3

Centers for Disease Control and Prevention (CDC). CDC statement on ACIP booster recommendations [Internet]. Atlanta (GA): U.S. Department of Health & Human Services; 2021 Sep 24 [cited 2021 Oct 18]. Available from: https://www.cdc.gov/media/releases/2021/p0924-booster-recommendations-.html.

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Footnote 4

Joint Committee on Vaccination and Immunisation (JCVI). JCVI statement regarding a COVID-19 booster vaccine programme for winter 2021 to 2022 [Internet]. London (United Kingdom): Department of Health & Social Care; 2021 Sep 14 [cited 2021 Oct 18]. Available from: https://www.gov.uk/government/publications/jcvi-statement-september-2021-covid-19-booster-vaccine-programme-for-winter-2021-to-2022/jcvi-statement-regarding-a-covid-19-booster-vaccine-programme-for-winter-2021-to-2022.

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Footnote 5

Covid-19 : la HAS élargit le périmètre de la dose de rappel [Internet]. Saint-Denis (France): Haute Autorité de santé (HAS); 2021 Oct 6 [cited 2021 Oct 18]. Available from: https://www.has-sante.fr/jcms/p_3290677/fr/covid-19-la-has-elargit-le-perimetre-de-la-dose-de-rappel.

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Footnote 6

[STIKO press release on the COVID-19 booster vaccination and the optimization of the Janssen basic immunization] [Internet]. Berlin: Robert Koch Institute; 2021 Oct 7 [cited 2021 Oct 18]. Available from: https://www.rki.de/DE/Content/Kommissionen/STIKO/Empfehlungen/PM_2021-10-07.html.

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Footnote 7

Petricic S. Israelis rush to get boosters in mass campaign for 3rd COVID-19 shots [Internet]. Toronto (ON): CBC News; 2021 Oct 6 [cited 2021 Oct 18]. Available from: https://www.cbc.ca/news/world/israel-covid-19-vaccine-booster-campaign-1.6200726.

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Footnote 8

Archived 12: NACI rapid response: Interchangeability of authorized COVID-19 vaccines [Internet]. Ottawa (ON): Public Health Agency of Canada; 2021 Jun 1 [cited 2021 Oct 18]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/recommendations-use-covid-19-vaccines/rapid-response-interchangeability.html#_Recommendations.

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Footnote 9

Archived 10: Extended dose intervals for COVID-19 vaccines to optimize early vaccine rollout and population protection in Canada in the context of limited vaccine supply [Internet]. Ottawa (ON): Public Health Agency of Canada; 2021 Apr 7 [cited 2021 Oct 18]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/extended-dose-intervals-covid-19-vaccines-early-rollout-population-protection.html.

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Footnote 10

El Sahly HM, Baden LR, Essink B, Doblecki-Lewis S, Martin JM, Anderson EJ, et al. Efficacy of the mRNA-1273 SARS-CoV-2 vaccine at completion of blinded phase. N Engl J Med. 2021 Sep 22. doi: 10.1056/NEJMoa2113017.

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Footnote 11

NACI rapid response: Booster dose in long-term care residents and seniors living in other congregate settings [Internet]. Ottawa (ON): Public Health Agency of Canada; 2021 Sep 28 [cited 2021 Oct 18]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/statement-september-28-2021-booster-dose-long-term-care-residents-seniors-living-other-congregate-settings.html.

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Footnote 12

Archived: National Advisory Committee on Immunization (NACI). Guidance on the prioritization of key populations for COVID-19 immunization [Internet]. Ottawa (ON): Public Health Agency of Canada; 2021 Feb 12 [cited 2021 Sep 21]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/guidance-prioritization-key-populations-covid-19-vaccination.html.

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Footnote 13

Ismail SJ, Tunis MC, Zhao L, Quach C. Navigating inequities: a roadmap out of the pandemic. BMJ Glob Health 2021 Jan;6(1):e004087. doi: 10.1136/bmjgh-2020-004087.

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Footnote 14

Zhao L, Ismail SJ, Tunis MC. Ranking the relative importance of COVID-19 vaccination strategies in Canada: a priority-setting exercise. CMAJ [doi: 10.9778/cmajo.20200241]. 2021 Sep 7 07/01;9(3):E848.

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Footnote 15

Archived: Guidance on the prioritization of initial doses of COVID-19 vaccine(s) [Internet]. Ottawa (ON): Public Health Agency of Canada; 2020 Dec 18 []. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/guidance-prioritization-initial-doses-covid-19-vaccines.html.

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Footnote 16

Archived. Preliminary guidance on key populations for early COVID-19 immunization [Internet]. Ottawa (ON): Public Health Agency of Canada; 2020 Nov 3 [cited 2021 Oct 18]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/guidance-key-populations-early-covid-19-immunization.html.

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Footnote 17

Ismail SJ, Zhao L, Tunis MC, Deeks SL, Quach C, National Advisory Committee on Immunization. Key populations for early COVID-19 immunization: preliminary guidance for policy. CMAJ. 2020 Nov 30;192(48):E1620,E1632. doi: 10.1503/cmaj.202353.

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Footnote 18

Ismail SJ, Langley JM, Harris TM, Warshawsky BF, Desai S, FarhangMehr M. Canada's National Advisory Committee on Immunization (NACI): Evidence-based decision-making on vaccines and immunization. Vaccine. 2010;28:A58,63. doi: 10.1016/j.vaccine.2010.02.035.

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Footnote 19

Ismail SJ, Hardy K, Tunis MC, Young K, Sicard N, Quach C. A framework for the systematic consideration of ethics, equity, feasibility, and acceptability in vaccine program recommendations. Vaccine. 2020 Aug 10;38(36):5861,5876. doi: 10.1016/j.vaccine.2020.05.051.

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Footnote 20

McDonald I, Murray SM, Reynolds CJ, Altmann DM, Boyton RJ. Comparative systematic review and meta-analysis of reactogenicity, immunogenicity and efficacy of vaccines against SARS-CoV-2. NPJ Vaccines. 2021 May 13;6(1):74. doi: 10.1038/s41541-021-00336-1.

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Footnote 21

Parry H, Bruton R, Stephens C, Brown K, Amirthalingam G, Hallis B, et al. Extended interval BNT162b2 vaccination enhances peak antibody generation in older people. medRxiv. 2021 May 17. doi: 10.1101/2021.05.15.21257017.

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Footnote 22

Amirthalingam G, Bernal JL, Andrews NJ, Whitaker H, Gower C, Stowe J, et al. Higher serological responses and increased vaccine effectiveness demonstrate the value of extended vaccine schedules in combatting COVID-19 in England. medRxiv. 2021 Jul 28. doi: 10.1101/2021.07.26.21261140.

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Footnote 23

Goel RR, Painter MM, Apostolidis SA, Mathew D, Meng W, Rosenfeld AM, et al. mRNA vaccines induce durable immune memory to SARS-CoV-2 and variants of concern. Science. 2021 Oct 14:eabm0829. doi: 10.1126/science.abm0829.

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Footnote 24

Turner JS, O'Halloran JA, Kalaidina E, Kim W, Schmitz AJ, Zhou JQ, et al. SARS-CoV-2 mRNA vaccines induce persistent human germinal centre responses. Nature. 2021 Aug;596(7870):109,113. doi: 10.1038/s41586-021-03738-2.

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Footnote 25

Public Health Agency of Canada. Canada COVID-19 weekly epidemiology report. 03 October to 09 October 2021 (week 40) [Internet]. Ottawa (ON): Government of Canada; 2021 Oct 15 [cited 2021 Oct 18]. Available from: https://www.canada.ca/content/dam/phac-aspc/documents/services/diseases/2019-novel-coronavirus-infection/surv-covid19-weekly-epi-update-20211015-en.pdf.

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Footnote 26

Indigenous Services Canada. Confirmed cases of COVID-19 [Internet]. Ottawa (ON): Government of Canada; 2021 Oct 18 [cited 2021 Oct 18]. Available from: https://www.sac-isc.gc.ca/eng/1598625105013/1598625167707.

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Footnote 27

Andrews N, Tessier E, Stowe J, Gower C, Kirsebom F, Simmons R, et al. Vaccine effectiveness and duration of protection of Comirnaty, Vaxzevria and Spikevax against mild and severe COVID-19 in the UK. medRxiv. 2021 Sep 21. doi: 10.1101/2021.09.15.21263583.

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Footnote 28

Kwong J. The effectiveness of COVID-19 vaccines in Ontario [recorded presentation]. ICES/CEU Rounds [Internet]. Toronto (ON): Sunnybrook Hospital; 2021 Oct 5 [cited 2021 Oct 18]. Available from: https://www.youtube.com/watch?v=rBHv0RgSYT8&list=PLBA654E5A75EBBBFE&index=2.

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Footnote 29

Bacon SL, Ribeiro PAB, Stojanovic J, Joyal-Desmarais K,; Marques Vieira A, Yip D, on behalf of the META Group. Change in the level of vaccine protection over time in COVID-19 vaccinated individuals: a rapid review [Internet]. Montreal: Montreal Behavioural Medicine Centre; 2021 Oct 8 [cited 2021 Oct 18]. Available from: https://www.mcmasterforum.org/docs/default-source/product-documents/rapid-responses/change-in-the-level-of-vaccine-protection-over-time-in-covid-19-vaccinated-individuals.pdf?sfvrsn=48d5b776_5.

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Footnote 30

Chemaitelly H, Tang P, Hasan MR, AlMukdad S, Yassine HM, Benslimane FM, et al. Waning of BNT162b2 vaccine protection against SARS-CoV-2 infection in Qatar. N Engl J Med. 2021 Oct 6. doi: doi: 10.1056/NEJMoa2114114.

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Footnote 31

Tartof SY, Slezak JM, Fischer H, Hong V, Ackerson BK, Ranasinghe ON, et al. Effectiveness of mRNA BNT162b2 COVID-19 vaccine up to 6 months in a large integrated health system in the USA: a retrospective cohort study. Lancet. 2021 Oct 16;398(10309):1407,1416. doi: 10.1016/S0140-6736(21)02183-8.

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Footnote 32

Bruxvoort K, Tseng HF, Qian L, Ackerson BK, Luo Y, Lee GS, et al. Real-world effectiveness of the mRNA-1273 vaccine against COVID-19: interim results from a prospective observational cohort study. SSRN Preprint. 2021 Sep 2. doi:10.2139/ssrn.3916094.

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Footnote 33

Sadoff J, Gray G, Vandebosch A, Cárdenas V, Shukarev G, Grinsztejn B, et al. Safety and efficacy of single-dose Ad26.COV2.S vaccine against Covid-19. N Engl J Med. 2021 Jun 10;384(23):2187,2201. doi: 10.1056/NEJMoa2101544.

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Footnote 34

Thomas SJ, Moreira ED,Jr, Kitchin N, Absalon J, Gurtman A, Lockhart S, et al. Safety and efficacy of the BNT162b2 mRNA Covid-19 vaccine through 6 months. N Engl J Med. 2021 Sep 15. doi: 10.1056/NEJMoa2110345.

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Footnote 35

Thompson MG, Stenehjem E, Grannis S, Ball SW, Naleway AL, Ong TC, et al. Effectiveness of Covid-19 vaccines in ambulatory and inpatient care settings. N Engl J Med. 2021 Oct 7;385(15):1355,1371. doi: 10.1056/NEJMoa2110362.

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Footnote 36

Parry H, Bruton R, Stephens C, Brown K, Amirthalingam G, Otter A, et al. Differential immunogenicity of BNT162b2 or ChAdOx1 vaccines after extended-interval homologous dual vaccination in older people. Immun Ageing. 2021 Aug 20;18(1):34. doi: 10.1186/s12979-021-00246-9.

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Footnote 37

Hall V, Ferreira V, Ierullo M, Ku T, Beata Majchrzak-Kita, Kulasingam V, et al. Delayed interval BNT162b2 mRNA COVID-19 vaccination provides robust immunity. Research Square [preprint]. 2021 Aug 24. doi: 10.21203/rs.3.rs-793234/v1.

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Footnote 38

International Vaccine Access Center, Johns Hopkins Bloomberg School of Public Health, World Health Organization. Results of COVID-19 vaccine effectiveness studies: an ongoing systematic review. Weekly summary tables. [Internet]. Baltimore (MD): International Vaccine Access Center; 2021 Oct 14 [cited 2021 Oct 18]. Available from: https://view-hub.org/sites/default/files/2021-10/COVID19%20Vaccine%20Effectiveness%20Transmission%20%20Impact%20Studies%20-%20Summary%20Tables_20211014.pdf.

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Footnote 39

Self WH, Tenforde MW, Rhoads JP, Gaglani M, Ginde AA, Douin DJ, et al. Comparative effectiveness of Moderna, Pfizer-BioNTech, and Janssen (Johnson & Johnson) vaccines in preventing COVID-19 hospitalizations among adults without immunocompromising conditions - United States, March-August 2021. MMWR Morb Mortal Wkly Rep. 2021 Sep 24;70(38):1337,1343. doi: 10.15585/mmwr.mm7038e1.

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Footnote 40

Nordström P, Ballin M, Nordström A. Effectiveness of heterologous ChAdOx1 nCoV-19 and mRNA prime-boost vaccination against symptomatic Covid-19 infection in Sweden: A nationwide cohort study. Lancet Reg Health Eur. 2021 Oct 17. doi: 10.1016/j.lanepe.2021.100249.

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Footnote 41

Puranik A, Lenehan PJ, Silvert E, Niesen MJM, Corchado-Garcia J, O'Horo JC, et al. Comparison of two highly-effective mRNA vaccines for COVID-19 during periods of Alpha and Delta variant prevalence. medRxiv. 2021 Aug 9. doi: 10.1101/2021.08.06.21261707.

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Footnote 42

Salmon C, Flanagan J, Farkas B, Mastikhina L, Egunsola O, Saxinger L, et al on behalf of the University of Calgary Health Technology Assessment Unit. Transmissibility of COVID-19 among vaccinated individuals: a rapid literature review, update #2 [Internet]. Toronto (ON): SPOR Evidence Alliance; 2021 Sep 24 [cited 2021 Oct 18]. Available from: https://sporevidencealliance.ca/wp-content/uploads/2021/10/Transmissibility-of-COVID-Vaccinated-Individuals_Final-Report_2021.09.24.pdf.

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Footnote 43

Levine-Tiefenbrun M, Yelin I, Alapi H, Katz R, Herzel E, Kuint J, et al. Viral loads of Delta-variant SARS-CoV2 breakthrough infections following vaccination and booster with the BNT162b2 vaccine. medRxiv. 2021 Sep 1. doi: 10.1101/2021.08.29.21262798.

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Footnote 44

Eyre DW, Taylor D, Purver M, Chapman D, Fowler T, Pouwels KB, et al. The impact of SARS-CoV-2 vaccination on Alpha & Delta variant transmission. medRxiv. 2021 Oct 15. doi: 10.1101/2021.09.28.21264260:2021.09.28.21264260.

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Footnote 45

de Gier B, Andeweg S, Backer JA, RIVM COVID-19 surveillance and,epidemiology team, Hahné SJM, van den Hof S, et al. Vaccine effectiveness against SARS-CoV-2 transmission to household contacts during dominance of Delta variant (B.1.617.2), August-September 2021, the Netherlands. medRxiv. 2021 Oct 14. doi: 10.1101/2021.10.14.21264959.

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Footnote 46

National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases. Science brief: COVID-19 vaccines and vaccination [Internet]. Atlanta (GA): United States Centers for Disease Control and Prevention (CDC); 2021 Sep 15 [cited 2021 Oct 18]. Available from: https://www.cdc.gov/coronavirus/2019-ncov/science/science-briefs/fully-vaccinated-people.html.

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Footnote 47

Pfizer Inc. BNT162b2 [COMIRNATY® (COVID-19 Vaccine, mRNA)] booster (third) dose [slides presented at Advisory Committee on Immunization Practices (ACIP) meeting] [Internet]. Atlanta (GA): United States Centers for Disease Control and Prevention; 2021 Sep 22 [cited 2021 Oct 18]. Available from: https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2021-09-22/02-COVID-Gruber-508.pdf.

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Footnote 48

Chu L, Montefiori D, Huang W, Nestorova B, Chang Y, Carfi A, et al. Immune memory response after a booster injection of mRNA-1273 for severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). medRxiv. 2021 Oct 1. doi: 10.1101/2021.09.29.21264089.

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Footnote 49

Moderna Inc. Safety and immunogenicity of a 50 µg booster dose of mRNA-1273 (Moderna COVID-19 Vaccine) [slides presented at Vaccines and Related Biological Products Advisory Committee meeting] [Internet]. Silver Spring (MD): United States Food and Drug Administration; 2021 Oct 14 [cited 2021 Oct 18]. Available from: https://www.fda.gov/media/153089/download.

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Footnote 50

Lee J. FDA review of effectiveness and safety of COMIRNATY (COVID-19 Vaccine, mRNA) booster dose biologics license application supplement [slides presented at Vaccines and Related Biological Products Advisory Committee Meeting] [Internet]. Silver Spring (MD): United States Food and Drug Administration; 2021 Sep 17 [cited 2021 Oct 18]. Available from: https://www.fda.gov/media/152239/download.

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Footnote 51

Israeli Ministry of Health, Weizmann Institute of Science, Gertner Institute, Hebrew University & Technion. Booster protection across ages - data from Israel [slides presented at Vaccines and Related Biological Products Advisory Committee Meeting] [Internet]. Silver Spring (MD): United States Food and Drug Administration; 2021 Oct 14 [cited 2021 Oct 18]. Available from: https://www.fda.gov/media/153086/download.

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Footnote 52

Bar-On YM, Goldberg Y, Mandel M, Bodenheimer O, Freedman L, Kalkstein N, et al. Protection of BNT162b2 vaccine booster against Covid-19 in Israel. N Engl J Med. 2021 Sep 15. doi: 10.1056/NEJMoa2114255.

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Footnote 53

Bar-On Y, Goldberg Y, Mandel M, Bodenheimer O, Freedman L, Alroy-Preis S, et al. Protection across age groups of BNT162b2 vaccine booster against Covid-19. medRxiv. 2021 Oct 7. doi: 10.1101/2021.10.07.21264626.

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Footnote 54

Patalon T, Gazit S, Pitzer VE, Prunas O, Warren JL, Weinberger DM. Short term reduction in the odds of testing positive for SARS-CoV-2; a comparison between two doses and three doses of the BNT162b2 vaccine. medRxiv. 2021 Aug 31. doi: 10.1101/2021.08.29.21262792.

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Footnote 55

COV-BOOST. A randomised, phase II UK multi-centre study to determine reactogenicity and immunogenicity of booster vaccination against ancestral and novel variants of SARS-CoV-2 [study protocol] [Internet]. Southampton: University Hospital Southampton NHS Foundation Trust; 2021 Jun 14 [cited 2021 Sep 22]. Available from: https://www.covboost.org.uk/files/cov-boostprotocolv3214jun2021cleanpdf.

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Footnote 56

NIH clinical trial evaluating mixed COVID-19 vaccine schedules begins [Internet]. Bethesda (MD): National Institutes of Health; 2021 Jun 1 [cited 2021 Oct 18]. Available from: https://www.nih.gov/news-events/news-releases/nih-clinical-trial-evaluating-mixed-covid-19-vaccine-schedules-begins.

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Footnote 57

Atmar RL, Lyke KE, Deming ME, Jackson LA, Branche AR, El Sahly HM, et al. Heterologous SARS-CoV-2 booster vaccinations – preliminary report. medRxiv. 2021 Oct 15. doi: 10.1101/2021.10.10.21264827.

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Footnote 58

Public Health Ethics Consultative Group (PHECG). Ethics consultation report. Advice provided to the Executive Secretary to the National Advisory Committee on Immunization (NACI) on the ethics of recommending COVID-19 vaccine boosters in absence of vaccine effectiveness data or evidence of vaccine failure. [Internal report]. Ottawa (ON): Public Health Agency of Canada; 2021 Sep 3.

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Footnote 59

Public Health Ethics Consultative Group (PHECG). Ethics consultation report. Advice to Executive Secretary to the National Advisory Committee on Immunization (NACI) on the ethics of recommending COVID-19 vaccine boosters for recipients of viral vector COVID-19 vaccines. [Internal report]. Ottawa (ON): Public Health Agency of Canada; 2021 Sep 27.

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Footnote 60

WHO Director-General's opening remarks at the media briefing on COVID-19 [Internet]. Geneva: World Health Organization; 2021 Sep 8 [cited 2021 Oct 21]. Available from: https://www.who.int/director-general/speeches/detail/who-director-general-s-opening-remarks-at-the-media-briefing-on-covid-19---8-september-2021.

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Footnote 61

Association for Canadian Studies (ACS). Eight in ten Canadians would be willing to take a boost against COVID-19 within the next year [Internet]. Montreal: ACS; 2021 Aug 17 [cited 2021 Oct 21]. Available from: https://acs-aec.ca/wp-content/uploads/2021/08/Vaccine-Boosterism.pdf.

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Footnote 62

Communications and Public Affairs Branch. COVID-19 tracking survey on Canadians' views, Wave 8, Aug 31-Sept 14, 2021 [internal report]. Ottawa (ON): Public Health Agency of Canada; 2021.

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Footnote 63

Ipsos. Global attitudes on COVID-19 vaccine booster shots [Internet]. Paris: Ipsos; 2021 Aug [cited 2021 Oct 21]. Available from: https://www.ipsos.com/sites/default/files/ct/news/documents/2021-09/Global-attitudes-about-COVID-19-Vaccine-Booster-Shots-Sept-2021.pdf.

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Footnote 64

Leger. North American Tracker [Internet]. Montreal (QC): Leger Marketing Inc; 2021 Aug 19 [cited 2021 Oct 21]. Available from: https://2g2ckk18vixp3neolz4b6605-wpengine.netdna-ssl.com/wp-content/uploads/2021/08/Legers-North-American-Tracker-August-19th-2021.pdf.

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Footnote 65

National Advisory Committee on Immunization (NACI) rapid response: Additional dose of COVID-19 vaccine in immunocompromised individuals following 1- or 2- dose primary series [Internet]. Ottawa (ON): Public Health Agency of Canada; 2021 Sep 10 [cited 2021 Oct 21]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/statement-september-10-2021-additional-dose-covid-19-vaccine-immunocompromised-following-1-2-dose-series.html#a5.

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Footnote 66

Kang M, Xin H, Yuan J, Ali ST, Liang Z, Zhang J, et al. Transmission dynamics and epidemiological characteristics of Delta variant infections in China. medRxiv. 2021 Aug 13. doi: 10.1101/2021.08.12.21261991.

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Footnote 67

Reports of suspected adverse reactions to COVID-19 vaccines in Switzerland – update [Internet]. Bern (Switzerland): Swissmedic; 2021 Sep 24 [cited 2021 Oct 16]. Available from: https://www.swissmedic.ch/swissmedic/en/home/news/coronavirus-covid-19/covid-19-vaccines-safety-update-6.html.

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Footnote 68

Medicines and Healthcare products Regulatory Agency (MHRA). Coronavirus vaccine - weekly summary of Yellow Card reporting [Internet]. London (United Kingdom): Department of Health and Social Care; 2021 Oct 7 [cited 2021 Oct 16]. Available from: https://www.gov.uk/government/publications/coronavirus-covid-19-vaccine-adverse-reactions/coronavirus-vaccine-summary-of-yellow-card-reporting.

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Footnote 69

Wingert A, Pillay J, Gates M, Guitard S, Rahman S, Beck A, et al. Risk factors for severity of COVID-19: a rapid review to inform vaccine prioritisation in Canada. BMJ Open. 2021 May 13;11(5):e044684. doi: 10.1136/bmjopen-2020-044684.

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Footnote 70

Statistics Canada. Custom data table (population with at least one underlying health condition), based on Canadian Community Health Survey, 2017-2018. Date received: 2021 January 18.

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Footnote 71

Department of Justice Canada. Fact sheet: Bill C-15 – United Nations Declaration on the Rights of Indigenous Peoples Act [Internet]. Ottawa (ON): Government of Canada; 2021 Jun 01 [cited 2021 Oct 21]. Available from: https://www.justice.gc.ca/eng/declaration/fact-fiche.html.

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Endnotes

Endnotes 1

Moderately to severely immunosuppressed includes individuals with the following conditions:

  • Active treatment for solid tumour or hematologic malignancies
  • Receipt of solid-organ transplant and taking immunosuppressive therapy
  • Receipt of chimeric antigen receptor (CAR)-T-cell therapy or hematopoietic stem cell transplant (within 2 years of transplantation or taking immunosuppression therapy)
  • Moderate to severe primary immunodeficiency (e.g., DiGeorge syndrome, Wiskott-Aldrich syndrome)
  • Stage 3 or advanced untreated HIV infection and those with acquired immunodeficiency syndrome
  • Active treatment with the following categories of immunosuppressive therapies: anti-B cell therapies (monoclonal antibodies targeting CD19, CD20 and CD22), high-dose systemic corticosteroids (refer to the CIG for suggested definition of high dose steroids), alkylating agents, antimetabolites, or tumor-necrosis factor (TNF) inhibitors and other biologic agents that are significantly immunosuppressive.

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