NACI updated guidance on booster COVID-19 vaccine doses in Canada

Publication: December 3, 2021

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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.

Introduction

On October 29, 2021, NACI published interim guidance on booster COVID-19 vaccine doses in Canada. Since NACI's initial guidance:

NACI has reviewed the evolving situation and evidence and has updated evidence-informed recommendations in this new context.

While the term "booster dose" is used in this guidance, NACI continues to monitor the emerging scientific data on whether this dose is indeed a booster dose (to stimulate the memory response once protection has truly waned), or should be considered part of the primary series (to establish strong immune response and memory). NACI will adjust the terminology as required.

NACI continues to recommend a primary COVID-19 vaccine series with an authorized mRNA vaccine in all authorized age groups, and continues to recommend that immunization in those who are eligible but who have not yet received their primary series should remain the top priority. NACI acknowledges the urgency for vaccinating people around the world who have not yet received any COVID-19 vaccine or completed their primary series.

COVID-19 vaccines have been shown to be very effective against symptomatic laboratory confirmed SARS-CoV-2 infection, severe disease, hospitalization, and death from COVID-19 to date. Incidence rates of new SARS-CoV-2 infection as well as rates of hospitalization, ICU admission, and mortality continue to be highest among unvaccinated individuals. In addition, those who have been vaccinated are less likely to become infected, and therefore are less likely to transmit SARS-CoV-2 infection to others.

However, in the context of the circulating Delta (B.1.617.2) variant, evidence is emerging that VE against SARS-CoV-2 infection and COVID-19 decreases with time after the primary series and there may be some decrease in protection against severe illness (especially in older individuals). Decreasing protection against infection could contribute to increased transmission, since infected individuals may be a source of infection for others. Therefore, a booster dose may provide more durable protection to reduce infection, transmission, and in some populations, severe disease.

Methods

The evidence pertaining to the need for and benefit of COVID-19 booster doses is rapidly evolving. NACI reviewed this evidence on November 30, 2021 and approved these updated recommendations on December 2, 2021. To date, NACI has published the following evidence-informed guidance on booster doses:

  1. Interim guidance on booster COVID-19 vaccine doses in Canada (October 29, 2021) including a description of NACI's decision-making framework
  2. Rapid response: Booster dose of COVID-19 vaccine in long-term care residents and seniors living in other congregate settings (September 28, 2021)

The intent of a "booster dose" is to restore protection that may have decreased over time or is no longer sufficient in individuals who initially responded adequately to a complete primary vaccine series. This is distinguished from the intent of an "additional dose" that might be added to the standard primary vaccine series with the aim of enhancing the immune response and establishing an adequate level of durable protection. NACI has also issued the following evidence-informed guidance for an "additional dose" in the primary series for moderately to severely immunocompromised individuals who may not have mounted an adequate immune response after a standard primary series:

  1. Rapid response: Additional dose of COVID-19 vaccine in immunocompromised individuals following 1- or 2- dose primary series (September 10, 2021)

NACI recommendations on the use of COVID-19 vaccines is 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 elsewhereFootnote 1Footnote 2 .

Recommendations

Please see Table 3 for an explanation of strong versus discretionary NACI recommendations.

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

Additional details are available in the following statements:

NACI's evidence-informed recommendations for booster doses of COVID-19 vaccines in adults 18 years of age and older

NACI acknowledges that the epidemiology of COVID-19 (including the impact of SARS-CoV-2 VoC) and the evidence on booster doses of COVID-19 vaccines are rapidly evolving, and continues to monitor the evidence in the Canadian context and provide additional recommendations as needed. NACI has therefore increased the strength of the recommendations from its interim guidance on October 29, 2021 and expanded the groups for whom booster doses are now recommended.

NACI makes the following evidence-informed recommendations on booster doses of authorized mRNA COVID-19 vaccines based upon emerging evidence on VE, the risks of exposure to SARS-CoV-2 in Canada at this time, the revised objectives of Canada's COVID-19 immunization program, the ongoing risk of severe illness from COVID-19, the societal disruption that results from transmission of infections, and the adverse impacts on health system capacity of the COVID-19 pandemic.

(Please refer to the section on "Options for vaccine type and dose offered for COVID-19 vaccine booster doses" in the context of these recommendations):

  1. NACI recommends that a booster dose of an authorized mRNA COVID-19 vaccine * should be offered ≥6 months after completion of a primary COVID-19 vaccine series to adults in the following populations:
    • Adults ≥50 years of age
    • Adults living in long-term care homes for seniors or other congregate living settings that provide care for seniors
    • Recipients of a viral vector vaccine primary series that was completed with only viral vector vaccines (AstraZeneca/COVISHIELD or Janssen COVID-19 vaccine)
    • Adults in or from First Nations, Métis and Inuit communities
    • Adults who are frontline healthcare workers (having direct close physical contact with patients) regardless of the interval between doses in their primary series

    (Strong NACI Recommendation)

  2. NACI recommends that a booster dose of an authorized mRNA COVID-19 vaccine * may be offered ≥6 months after completion of a primary COVID-19 vaccine series to adults 18-49 years of age with consideration of jurisdictional and individual risks.

    (Discretionary NACI Recommendation)

The relative need for a booster dose varies by a number of factors that may differ between jurisdictions and between individuals as summarized in Table 1.

Table 1. Considerations to determine the need for a booster dose of COVID-19 vaccine
Factor Assessment of Considerations
Jurisdictional Local epidemiology
  • Circulation of virus, including VoC
  • Evidence of decreasing protection against severe disease, infection, transmission
Health system capacity and access
  • Limited health system capacity to withstand a surge in cases
  • Reduced access to health care
Vaccine coverage of primary series in the population
  • Lower vaccine coverage at a regional population level leads to lower indirect protection and higher risk of breakthrough infection
Individual Risk of increased waning of protection and/or less protection
  • Shorter interval between doses in the primary series
  • Longer time since completion of primary series
  • Moderately to severely immunocompromised individuals
  • Vaccination with only viral vector vaccines
Risk of severe illness from COVID-19
  • Older age
  • Underlying medical condition (including those who are immunocompromised and who received a three-dose primary series)
  • Racialized and marginalized populations who have been disproportionately affected due to a number of intersecting equity factors
Risk of transmission to individuals at increased risk of severe illness from COVID-19
  • Close contact with those at risk for severe disease (e.g., healthcare provider, primary caregiver)
  • Decreased ability to physically distance (e.g., congregate living settings)
  • Decreased access to infection prevention and control measures

Options for vaccine type and dose offered for COVID-19 vaccine booster doses

NACI has indicated circumstances where specific products and/or doses may be preferred for a booster/additional dose as outlined in Table 2. However, if any of Pfizer-BioNTech Cormirnaty (30 mcg), Moderna Spikevax (50 mcg) or Moderna Spikevax (100 mcg) are administered as a booster/additional dose, the dose should be considered valid and therefore would not need to be repeated.

Table 2. OptionsFootnote a and considerations for vaccine types and doses offered for COVID-19 vaccine booster doses for certain populations
Population Vaccine type (and dose) for booster doses which may be preferred Rationale or additional considerations
18 to 29 year olds Pfizer-BioNTech Comirnaty (30 mcg)
  • Lower reported rates of myocarditis/pericarditis following vaccination with Pfizer-BioNTech Comirnaty (30 mcg) compared to Moderna Spikevax (100 mcg). [This is based upon data following the second dose in the primary series using Moderna Spikevax (100 mcg). Evidence following the Moderna Spikevax (50 mcg) booster dose is limited.]
  • ≥70 year olds
  • Adults living in long-term care homes for seniors or other congregate living settings that provide care for seniors
  • Moderately to severely immunocompromised adultsFootnote b (after the recommended 3-dose primary series)

Either Moderna Spikevax or Pfizer Comirnaty (30mcg) may be considered.

If Moderna Spikevax vaccine is being used as the booster product, a 100 mcg dose may be preferred. For moderately to severely immunocompromised adults, this is based on clinical discretion.

  • Moderna Spikevax (100 mcg) induces somewhat higher antibody levels compared to Pfizer-BioNTech Comirnaty (30 mcg)
  • Protection (against infection and severe disease) from a primary series with Moderna Spikevax (100 mcg) may be more durable than Pfizer-BioNTech Comirnaty (30 mcg)
  • These populations may have less robust immune function (elderly) or a diminished immune response to the vaccine (some immunocompromised individuals). It is possible that Moderna Spikevax (100 mcg) may induce a better immune response than Moderna Spikevax (50 mcg), although there is currently no direct comparison of these two dosages as boosters
  • Currently there are no data comparing the immune responses after a booster vaccination with Moderna Spikevax (100 mcg) and Pfizer-BioNTech Comirnaty (30 mcg) in these populations.
  • There is heterogeneity among those who are moderately to severely immunocompromised, and risks from COVID-19, as well as the likelihood of a reduced response to vaccines, will vary depending on age and the immunocompromising condition.
  • It should be noted that Moderna Spikevax (100 mcg) is not currently authorized by Health Canada as a booster dose.

For all other populations in whom booster doses are recommended that have not been specified above.

Either Moderna Spikevax (50 mcg) or Pfizer-BioNTech Comirnaty (30 mcg) are suitable products as a booster dose

  • Authorized as booster doses by Health Canada
Footnote a

A booster dose with a viral vector COVID-19 vaccine (AstraZeneca Vaxzevria or Janssen) should only be considered when other authorized COVID-19 vaccines are contraindicated or inaccessible. Viral vector vaccines are not currently authorized as booster doses in Canada. Vaccine effectiveness against symptomatic infection and severe COVID-19 outcomes has consistently been somewhat lower, and vaccine protection against infection and symptomatic disease decreases more quickly with viral vector vaccines compared to mRNA vaccines when used in a primary series. Viral vector vaccines also have a risk of vaccine-induced immune thrombotic thrombocytopenia (VITT) and other adverse effects that are not concerns with mRNA vaccines.

Return to footnote a referrer

Footnote b

Moderately or severely immunocompromised adults receiving a booster dose after a primary series of 3 doses, will receive a total of 4 doses.

Return to footnote b referrer

Additional considerations, summary of evidence and rationale

Further information, a summary of evidence, and ethical, equity, feasibility and acceptability considerations are available in NACI's Interim guidance on booster COVID-19 vaccine doses in Canada.

NACI is continuing to monitor the evidence and 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 3. 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 be offered for some populations/individuals in some circumstances. Alternative approaches may be reasonable.

Acknowledgments

This statement was prepared by : SJ Ismail, B Warshawsky, M Salvadori, E Wong, K Farrah, M Tunis, J Zafack, L Coward, C Jensen, R Krishnan, K Young, R Harrison and S Deeks on behalf of NACI.

NACI gratefully acknowledges the contribution of: N Forbes, J Montroy, K Ramotar, C Mauviel

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: L Bill (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 / N Ivers (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), 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, 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

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 2

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Footnote 3

Global Affairs Canada. Canada's international vaccine donations [Internet]. Ottawa (ON): Government of Canada; 2021 Nov 22 [cited 2021 Dec 2]. Available from: https://www.international.gc.ca/world-monde/issues_development-enjeux_developpement/global_health-sante_mondiale/vaccine_donations-dons_vaccins.aspx?lang=eng.

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Footnote 4

Bacon SL, Ribeiro PAB, Stojanovic J, Joyal-Desmarais K, Vieira AM, Yip D, et al. Change in the level of vaccine protection over time in COVID-19 vaccinated individuals: a rapid review [Internet]. Version 3. Montreal: Montreal Behavioural Medicine Centre; 2021 Nov 26 [cited 2021 Dec 2]. Available from: https://sporevidencealliance.ca/wp-content/uploads/2021/11/Waning-Vaccine-Effectiveness_Update-1-Report_2021.11.26.pdf

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Footnote 5

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 6

Goldberg Y, Mandel M, Bar-On YM, Bodenheimer O, Freedman L, Haas EJ, et al. Waning immunity after the BNT162b2 vaccine in Israel. N Engl J Med. 2021 Oct 27. doi: 10.1056/NEJMoa2114228.

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Footnote 7

Robles Fontán MM, Nieves EG, Gerena IC, Irizarry RA. Time-varying effectiveness of three Covid-19 vaccines in Puerto Rico. medRxiv. 2021 Oct 20. doi: 10.1101/2021.10.17.21265101.

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Footnote 8

Nunes B, Rodrigues AP, Kislaya I, Cruz C, Peralta-Santos A, Lima J, et al. mRNA vaccines effectiveness against COVID-19 hospitalizations and deaths in older adults: a cohort study based on data-linkage of national health registries in Portugal. medRxiv. 2021 Aug 29. doi: 10.1101/2021.08.27.21262731.

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Footnote 9

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 10

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 11

Feikin D, Higdon MM, Abu-Raddad LJ, Andrews N, Araos R, Goldberg Y, et al. Duration of effectiveness of vaccines against SARS-CoV-2 infection and COVID-19 disease: Results of a systematic review and meta-regression. SSRN Preprints. 2021 Nov 18. http://dx.doi.org/10.2139/ssrn.3961378.

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Footnote 12

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 13

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 14

Cohn BA, Cirillo PM, Murphy CC, Krigbaum NY, Wallace AW. SARS-CoV-2 vaccine protection and deaths among US veterans during 2021. Science. 2021 Nov 4:eabm0620. doi: 10.1126/science.abm0620.

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Footnote 15

Lin D, Gu Y, Wheeler B, Young H, Holloway S, Sunny SK, et al. Effectiveness of Covid-19 vaccines in the United States over 9 months: surveillance data from the state of North Carolina. medRxiv. 2021 Oct 26. doi: 10.1101/2021.10.25.21265304.

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Footnote 16

Skowronski DM, Setayeshgar S, Febriani Y, Ouakki M, Zou M, Talbot D, et al. Two-dose SARS-CoV-2 vaccine effectiveness with mixed schedules and extended dosing intervals: test-negative design studies from British Columbia and Quebec, Canada. medRxiv. 2021 Oct 26. doi: 10.1101/2021.10.26.21265397.

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Footnote 17

Rosenberg ES, Dorabawila V, Easton D, Bauer UE, Kumar J, Hoen R, et al. COVID-19 vaccine effectiveness by product and timing in New York State. medRxiv. 2021 Oct 9. doi: 10.1101/2021.10.08.21264595.

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Footnote 18

Ireland G, Whitaker H, Ladhani SN, Baawuah F, Subbarao V, Linley E, et al. Serological responses to COVID-19 booster vaccine in England. medRxiv. 2021 Nov 24. doi: 10.1101/2021.11.22.21266692.

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Footnote 19

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 20

Falsey AR, Frenck RW, Walsh EE, Kitchin N, Absalon J, Gurtman A, et al. SARS-CoV-2 neutralization with BNT162b2 vaccine dose 3. N Engl J Med. 2021 Oct 21;385(17):1627,1629. doi: 10.1056/NEJMc2113468.

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Footnote 21

Choi A, Koch M, Wu K, Chu L, Ma L, Hill A, et al. Safety and immunogenicity of SARS-CoV-2 variant mRNA vaccine boosters in healthy adults: an interim analysis. Nat Med. 2021 Nov;27(11):2025,2031. doi: 10.1038/s41591-021-01527-y.

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Footnote 22

Perez JL. Efficacy & safety of BNT162b2 booster -C4591031 2 month interim analysis [slides presented at Advisory Committee on Immunization Practices (ACIP) meeting November 19, 2021] [Internet]. Atlanta (GA): Centers for Disease Control and Prevention (CDC); 2021 Nov [cited 2021 Nov 30]. Available from: https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2021-11-19/02-COVID-Perez-508.pdf.

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Footnote 23

Andrews N, Stowe J, Kirsebom F, Gower C, Ramsay M, Bernal JL. Effectiveness of BNT162b2 (Comirnaty, Pfizer-BioNTech) COVID-19 booster vaccine against covid-19 related symptoms in England: test negative case-control study. medRxiv. 2021 Nov 15. doi: 10.1101/2021.11.15.21266341.

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Footnote 24

Saciuk Y, Kertes J, Shamir Stein N, Ekka Zohar A. Effectiveness of a third dose of BNT162b2 mRNA vaccine. J Infect Dis. 2021 Nov 2. doi: 10.1093/infdis/jiab556.

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Footnote 25

Barda N, Dagan N, Cohen C, Hernán MA, Lipsitch M, Kohane IS, et al. Effectiveness of a third dose of the BNT162b2 mRNA COVID-19 vaccine for preventing severe outcomes in Israel: an observational study. Lancet. 2021 Oct 29. doi: 10.1016/S0140-6736(21)02249-2.

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Footnote 26

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 27

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 28

Shimabukuro, T. COVID-19 vaccine booster dose safety [slides presented at Advisory Committee on Immunization Practices (ACIP) meeting November 19, 2021] [Internet]. Atlanta (GA): Centers for Disease Control and Prevention; 2021 Nov [cited 2021 Nov 29]. Available from: https://www.cdc.gov/vaccines/acip/meetings/downloads/slides-2021-11-19/04-COVID-Shimabukuro-508.pdf.

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Footnote 29

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 Nov 29]. Available from: https://www.fda.gov/media/153086/download.

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Footnote 30

Shimabukuro TT, Kim SY, Myers TR, Moro PL, Oduyebo T, Panagiotakopoulos L, et al. Preliminary findings of mRNA Covid-19 vaccine safety in pregnant persons. N Engl J Med. 2021 Jun 17;384(24):2273,2282. doi: 10.1056/NEJMoa2104983.

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Footnote 31

Munshi L, Wright JK, Zipursky J, Jorgensen S, Bogler T, Miller KJ, et al. The incidence, severity, and management of COVID-19 in critically ill pregnant individuals. Science Briefs of the Ontario COVID-19 Science Advisory Table. 2021;2(43):https://doi.org/10.47326/ocsat.2021.02.43.1.0.

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Footnote 32

Public Health Agency of Canada (PHAC). COVID-19: PHAC modelling group report [internal report]. Ottawa (ON): PHAC; 2021 Nov 25.

Return to footnote 32 referrer

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