Archived 36: Initial guidance on a second booster dose of COVID-19 vaccines in Canada [2022-04-05]

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Publication: April 5, 2022

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Organization: Public Health Agency of Canada

Published: 2022-04-05

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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.


How to optimally define primary series and "booster doses" for COVID-19 vaccines are still in evolution. While the term "booster dose" is used in this guidance, NACI continues to monitor the emerging scientific data and will adjust the terminology as required. In this guidance, the term "second booster dose" will refer to the dose given after the first booster dose which follows a complete primary series. For example, a second booster will correspond to a 4th dose among immunocompetent individuals as they have a recommended 2-dose primary series, while it will correspond to a 5th dose among moderately to severely immunocompromised individuals as they have a recommended 3-dose primary series.

NACI recommended a first booster dose for long-term care home (LTC) residents and seniors living in other congregate settings on September 28, 2021. On October 29, 2021, NACI recommended booster doses for adults 80 years of age and over, with a discretionary recommendation for those 70 to 79 years of age and some other groups. On December 3, 2021, NACI published updated guidance on booster COVID-19 vaccine doses in Canada for adults 18 years of age and older. These recommendations on booster doses of COVID-19 vaccines were reviewed and reaffirmed in the context of the emergence of Omicron (B.1.1.529) variant of concern (VOC) on December 14, 2021. Since that time:

NACI continues to recommend a primary series with an authorized mRNA vaccine in all authorized age groups and a first booster dose among adults (for whom a first booster is authorized) and among high-risk adolescents (for whom a first booster is currently off-label). Immunization of those who are eligible for vaccination but have not yet received a primary vaccine series remains a top priority in Canada.

NACI's recommendations remain aligned with the goals of the Canadian COVID-19 Pandemic Response that have been updated on February 14, 2022:


NACI's recommendations on booster doses are based on the decision-making framework outlined in the published statement entitled Interim guidance on booster COVID-19 vaccine doses in Canada. Recommendations are based on evidence of the need for (e.g., evidence of decreased vaccine effectiveness (VE) against severe illness and/or infection depending on the population) and benefit of (e.g., safety and effectiveness) booster doses in the Canadian context.

On March 1, 2022 and March 22, 2022 NACI reviewed emerging evidence on the duration of protection of the first booster dose and the VE and safety of a second booster dose of COVID-19 vaccines. NACI also reviewed data on the current epidemiology of COVID-19, risk factors associated with severe outcomes from COVID-19 among older adults living in the community, and residents of long-term care homes or other congregate living settings for seniors.

NACI approved these updated recommendations on March 31, 2022.

For further information on NACI's recommendations on the use of COVID-19 vaccines, please refer to the COVID-19 vaccine chapter in the Canadian Immunization Guide (CIG).

Further information on NACI's process and procedures is available elsewhereFootnote 1Footnote 2.


The NACI recommendations on a first booster dose in younger age groups, including adolescents 12 to17 years of age, are under review and an update will be forthcoming. For details on the current recommendations, consult the following statements: updated guidance on booster COVID-19 vaccine doses in Canada for adults 18 years of age and older, guidance on the use of booster COVID-19 vaccine doses in adolescents 12 to 17 years of age and timing of COVID-19 vaccination for individuals previously infected with SARS-CoV-2.

In addition, regarding a second booster dose, NACI now makes the following recommendations based on the current evidence, the objectives of the Canadian COVID-19 pandemic response, and the evolving epidemiology of the COVID-19 pandemic.

In the context of protection against severe disease potentially decreasing over time following the first booster dose, and/or risk of immune evasion by highly transmissible VOC which can cause severe disease:

  1. NACI recommends that jurisdictions prepare for the rapid deployment of a second COVID-19 vaccine booster dose program over the coming weeks prioritizing the following populations, as close surveillance and assessment suggest concerning trends in the COVID-19 pandemic:
    1. Adults 80 years of age and over living in the community
      (Strong NACI Recommendation)
    2. Residents of long-term care or other congregate living settings for seniors
      (Strong NACI Recommendation)
    3. While the greatest benefit is expected in adults 80 years of age and older, jurisdictions may also consider offering a second COVID-19 booster dose to adults 70 to79 years of age living in the community.
      (Discretionary NACI Recommendation)
      • Timely, close, ongoing monitoring and assessment of provincial and territorial, national and international data are important to determine possible triggers for rapid implementation of local or provincial and territorial COVID-19 vaccination campaigns along with public health measures.
      • In general, jurisdictions should aim to provide a second booster dose 6 months after receipt of the previous booster dose, but this 6 month interval may need to be balanced with local and current epidemiology. As a result, shorter intervals may be indicated for these older adult populations in these settings at this time. When applicable, timing of recent COVID-19 infection should also be considered.
      • A second booster dose among adults younger than 70 years of age in or from First Nations, Métis, or Inuit communities may be considered*. Autonomous decisions should be made by Indigenous Peoples with the support of healthcare and public health partners in accordance with the United Nations Declaration on the Rights of Indigenous Peoples.
      • Among adults 70 years of age and over living in the community and residents of long-term care homes or other congregate living settings for seniors, either Moderna Spikevax (50mcg) or Pfizer-BioNTech Comirnaty (30mcg) may be considered for second booster doses. The use of the Moderna Spikevax vaccine (100mcg) as the booster may also be considered based on clinical discretion. The Novavax Nuvaxovid COVID-19 vaccine may be offered as a booster dose to individuals who are unable or unwilling to receive an mRNA COVID-19 vaccine.

The planning and deployment of a second COVID-19 booster dose program** for other high-risk groups (such as key populations*** previously prioritized) and/or for the general population may also be relevant in the future if surveillance and assessment**** suggest concerning trends in the COVID-19 pandemic (e.g., emergence or re-emergence of VOC causing severe disease). Planning should take into account that vaccine deployment may be required for broader population groups in the fall of 2022 or earlier depending on the COVID-19 pandemic context.

NACI will continue to monitor the evidence and update its recommendations as needed.

*A second booster dose among adults younger than 70 years of age in or from First Nations, Métis, or Inuit communities may be considered as these communities have a younger age distribution but increased risk for severe disease due to a variety of intersecting factors including underlying medical conditions and potential decreased access to health care.

**Details on options and considerations for vaccine types among certain adult populations are available in Table 4 of the recommendations on the use of Medicago COVID-19 vaccine

*** NACI is currently reviewing the key populations and will provide further details in future statements

****Factors to consider in the surveillance and assessment for future booster doses may vary by jurisdiction and are summarized in Table 1 below (updated from NACI's Interim guidance on booster COVID-19 vaccine doses in Canada)

Additional Considerations

Table 1. Underlying factors for consideration to determine the need for and benefit of subsequent COVID-19 booster dose programs in various populations.
Underlying factors for consideration Evidence to review to determine the need for and benefit of a second booster dose of COVID-19 vaccine
Risk benefit analysis
  • Risk of severe illness and death (e.g. older age, underlying medical conditions, populations who have been disproportionately affected by the pandemic due to a number of intersecting equity factors)
  • Risk of exposure (e.g. inability to physically distance and lack or poor access to infection prevention and control measures, frontline healthcare providers; congregate living facilities)
  • Risk of transmission to vulnerable populations (e.g. caregivers of those at increased risk of severe illness and death)
  • Risk of societal disruption (e.g., frontline essential workers)
  • Prevention of multisystem inflammatory syndrome in children (MIS-C) and post-COVID-19 syndromes
COVID-19 epidemic conditions
  • Circulation of SARS-CoV-2 strain (re-emergence of a previous strain or emergence of new strain) in Canada or internationally
  • Breakthrough cases, outbreaks
  • Case rates, virulence and implications for health system capacity
  • Duration of protection for COVID-19 vaccines against infection and against severe outcomes of COVID-19
Population level cumulative immunity and vaccine coverage
  • Initial vaccination series (time since last dose, coverage, type, interval between doses)
  • Previous SARS-CoV-2 infection
Vaccine characteristics in different groups against strain circulating or expected to circulate
  • Duration of protection
  • Immunogenicity
  • Efficacy and effectiveness in preventing transmission, infection and severe disease
  • Safety
Vaccine types available and forecasted
  • Number and type of available vaccines (including availability of vaccines expected to be effective against circulating strains)
  • Possible future vaccines (e.g., new formulations effective against variants, new vaccine technologies, or new delivery routes)
  • Global equity and supply

Summary of Evidence

Evolving epidemiology

Coverage and protection from recent infection

VE over time following a first booster

VE following a second booster

Immunogenicity of first or second booster doses in older adults and long-term care (LTC) residents


Ethics, equity, feasibility, and acceptability (EEFA)

Other considerations

Research Priorities

  1. Continuous monitoring of data on the safety, immunogenicity, efficacy, and effectiveness of the COVID-19 vaccines, including booster doses, through clinical trials and studies in real-world settings, including with regard to the duration of protection from first booster doses and the impact and duration of protection from second booster doses.
  2. Further evaluations of the optimal interval between booster dose administration, as well as further evaluations of the optimal interval between previous SARS-CoV-2 infection and booster dose administration.
  3. Vigilant monitoring and reporting of adverse events of special interest, including myocarditis and/or pericarditis, in order to accurately inform potential risks associated with a future booster. Global collaboration should be prioritized to enable data sharing so decision makers around the world can weigh benefits and risks of second booster doses of COVID-19 vaccines.
  4. Continuous monitoring of COVID-19 epidemiology and VE in special populations (e.g. residents of long-term care facilities, adults of advanced age, individuals with high-risk medical conditions, pregnancy, and communities disproportionately affected by COVID-19) and across a range of clinical outcomes (i.e. post-COVID-19 syndromes and long-term consequences of COVID-19 infection; MIS-C; severe disease).
  5. Further evaluation on the optimal timing and trigger for the initiation of potential future booster dose recommendations, as well as evaluation of potential risks associated with providing booster doses earlier than necessary.
  6. Continuous monitoring of vaccine uptake in the Canadian population, particularly in the context of subsequent booster doses.
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"

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 be considered for some populations/individuals in some circumstances. Alternative approaches may be reasonable.


This statement was prepared by: J Zafack, B Warshawsky, N Forbes, SJ Ismail, J Montroy, A Nunn, R Krishnan, E Wong, R Stirling, M Salvadori, K Young, MC Tunis, B Sander, and R Harrison, on behalf of NACI.

NACI gratefully acknowledges the contribution of: N. St. Pierre, K Ramotar, S H Lim, E Terrataca, L Coward and C Jensen

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

Liaison representatives: L Bill / M Nowgesic (Canadian Indigenous Nurses Association), 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), D Fell (Canadian Association for Immunization Research and Evaluation), S Funnell (Indigenous Physicians Association of Canada), J Hu (College of Family Physicians of Canada), M Lavoie (Council of Chief Medical Officers of Health), D Moore (Canadian Paediatric Society), M Naus (Canadian Immunization Committee), A Ung (Canadian Pharmacists Association).

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), D MacDonald (COVID-19 Epidemiology and Surveillance, PHAC), 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), N Brousseau, Y-G Bui, S Deeks, K Dooling, K Hildebrand, M Miller, M Murti, J Papenburg, D Smith, and S Vaughan.

PHAC participants: NK Abraham, N Alluqmani, L Coward, N Forbes, C Jensen, CY Jeong, A Jirovec, A Killikelly, R Krishnan, SH Lim, N Mohamed, J Montroy, A Nam, S Pierre, R Pless, M Salvadori, A Sinilaite, A Stevens, R Stirling, E Tice, A Tuite, MC Tunis, B Warshawsky, E Wong, R Ximenes, MW Yeung, J Zafack.

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