Updated guidance on the use of COVID-19 vaccines in individuals who have not previously been vaccinated against COVID-19

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Organization: Public Health Agency of Canada

Date published: 2023-10-27

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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 8programmatic 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

On September 12 and 28, 2023, Health Canada authorized XBB.1.5-containing mRNA COVID-19 vaccine products (Moderna Spikevax XBB.1.5 and Pfizer-BioNTech Comirnaty Omicron XBB.1.5 respectively) for use in individuals 6 months of age and older (see Table 1 for authorized products). The updated COVID-19 vaccine has been authorized for use in those who have never previously received a COVID-19 vaccine (also referred to as a "primary series"), and those who have previously been vaccinated with any COVID-19 vaccines.

Interim guidance on the use of bivalent Omicron-containing COVID-19 vaccines for primary series was provided in June 2023. In the NACI Addendum to the guidance on the use of COVID-19 vaccines in the fall of 2023 issued in September 2023, NACI acknowledged the use of the XBB.1.5 formulations and provided interim guidance on the use of the XBB.1.5-containing vaccines as a primary series for those who had never been vaccinated with a COVID-19 vaccine or were only partially vaccinated, with further advice to follow. The complete additional advice is provided in this October 2023 statement.

Since 2022, NACI has strongly recommended a primary series of mRNA COVID-19 vaccination for everyone 5 years of age and older, with a discretionary recommendation for children 6 months to under 5 years of age. With the recent changes to the vaccine schedules for the XBB.1.5-containing COVID-19 vaccines as outlined in the product monographs, NACI is updating its guidance on the use of COVID-19 vaccines for individuals who have not been previously vaccinated.

Methods

On July 25, August 14, September 8 and 19, 2023, the NACI COVID-19 Working Group (WG) reviewed the available epidemiology and evidence on hybrid immunity, rates of myocarditis and/or pericarditis following immunization, vaccine effectiveness (VE) of bivalent vaccines, vaccine protection for moderately to severely immunocompromised individuals, and immunogenicity of XBB.1.5-containing vaccines compared to earlier formulations.

On September 28, 2023, NACI reviewed the evidence presented to the COVID-19 WG, reached consensus on proposed recommendations, and approved the Statement on October 20, 2023. An early briefing on the recommendations was provided by NACI to provinces and territories on October 5, 2023 in order to facilitate rollout of the new vaccine products.

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

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

Overview of evidence

Information available as of September 19, 2023 is summarized below.

Epidemiology

Vaccine protection

Vaccine protection in individuals with moderately to severely immunocompromising conditions

Risk of myocarditis and/or pericarditis following COVID-19 vaccination

Ethics, equity, feasibility, and acceptability (EEFA)

Recommendations

Please see Table 4 for an explanation of strong versus discretionary NACI recommendations.

For individuals 5 years of age and older who are previously unvaccinated:

  1. NACI recommends that individuals 5 years of age and older who are previously unvaccinated against COVID-19 should be vaccinated. The latest formulations of mRNA COVID-19 vaccines are recommended. (Strong NACI recommendation)
    • 1 dose is recommended as per the authorized schedule in the product monograph. An additional dose is recommended for individuals who are moderately to severely immunocompromised (see Recommendation #4).
    • For the number of doses and schedules, see Table 2 for those not previously vaccinated and Table 3 for those who started a primary series with a non-XBB.1.5 vaccine.

      Considerations:

    • The latest formulations of Health Canada authorized mRNA COVID-19 vaccines available as of fall 2023 are monovalent XBB.1.5, with vaccine products from Moderna (Spikevax) and Pfizer-BioNTech (Comirnaty). See Table 1 and Table 2 for dosages.
    • The recommendations for those 5 years of age and older take into account the high levels of seroprevalence due to infection in the population at this time.
    • There is no longer a product preference between Moderna Spikevax and Pfizer-BioNTech Comirnaty with the use of XBB.1.5-containing COVID-19 vaccines for unvaccinated individuals 12 to 29 years of age. Please see "Additional considerations" for more information.
    • The reduced dose schedule for unvaccinated individuals 5 years of age and older is applied going forward for those receiving the XBB.1.5-containing COVID-19 vaccine as their first dose.
      • Individuals 5 year of age and older who started a primary series with earlier formulations (i.e., original monovalent, original+BA.1 bivalent or original+BA.4/5 bivalent) but did not complete the series are recommended to complete the series with an XBB.1.5-containing vaccine based on the total number doses previously recommended (i.e., they should receive a total of 2 COVID-19 vaccine doses in the primary series if not immunocompromised and 3 doses in the primary series if they are moderately to severely immunocompromised). See Table 3.

For children 6 months to under 5 years of age who are previously unvaccinated:

  1. NACI recommends that children 6 months to under 5 years of age who are previously unvaccinated against COVID-19 and who are at high risk of severe illness due to COVID-19 should be vaccinated. The latest formulations of mRNA COVID-19 vaccines are recommended. (Strong NACI recommendation)
  2. NACI recommends that children 6 months to under 5 years of age who are previously unvaccinated against COVID-19 and who are not known to be at high risk of severe illness due to COVID-19 may be vaccinated. The latest formulations of mRNA COVID-19 vaccines are recommended. (Discretionary NACI recommendation)
    • Risk factors for severe illness due to COVID-19: There is limited evidence on clinical risk factors for severe COVID-19 disease in pediatric populations. Children at increased risk for severe outcomes may include children who are medically fragile/have medical complexities, children with more than one comorbidity, children with neurological disorders, children with chronic lung disease, and children with Down syndrome (Trisomy 21), and other immunocompromising conditions.
    • 2 doses of Moderna Spikevax or 3 doses of Pfizer-BioNTech Comirnaty are recommended, with an 8-week interval between doses. An additional dose is recommended for individuals who are moderately to severely immunocompromised (see Recommendation #4).
    • For the number of doses and schedules, see Table 2 for those not previously vaccinated and Table 3 for those who started a primary series with a non-XBB.1.5 vaccine.

      Considerations

    • The latest formulations of Health Canada authorized mRNA vaccines available as of fall 2023 are monovalent XBB.1.5, with vaccine products from Moderna (Spikevax) and Pfizer-BioNTech (Comirnaty). See Table 1 and Table 2 for dosages.
    • A mixed product schedule using vaccines from different manufacturers can be offered for the primary series; however, if both Pfizer-BioNTech Comirnaty and Moderna Spikevax vaccine products are used in the same primary series for an individual 6 months to under 5 years of age, the total number of doses in the series should follow the Pfizer-BioNTech Comirnaty schedule (see Table 3).
    • If children 6 months to under 5 years of age started the primary series with a non-XBB.1.5 vaccine but did not complete the series, they should complete the primary series with an XBB.1.5-containing vaccine (see Table 3).
    • Children who started the primary series at less than 5 years of age and turn 5 years of age before completing the series, should receive the number of XBB.1.5 vaccine doses recommended as per Table 3 for those 5 years of age and older and using the age-appropriate dosage for their current age.

For individuals 6 months of age and older who are moderately to severely immunocompromised and are previously unvaccinated:

  1. NACI recommends that individuals 6 months of age and older who are previously unvaccinated and are moderately to severely immunocompromised should receive an additional dose of COVID-19 vaccine above the number of doses recommended for those who are not immunocompromised. (Strong NACI recommendation)
    • For individuals 6 months to under 5 years of age who are moderately to severely immunocompromised, NACI recommends a schedule consisting of 3 doses of Moderna Spikevax (preferred) or 4 doses of Pfizer-BioNTech Comirnaty, with a 4- to 8-week interval between doses. Product preference for children 6 months to under 5 years of age who are moderately to severely immunocompromised: 3 doses of the Moderna Spikevax (25 mcg) vaccine is preferred over 4 doses of Pfizer-BioNTech Comirnaty (3 mcg) because there is likely higher acceptability and more feasible implementation due to fewer doses in the schedule using Moderna Spikevax.
    • For individuals 5 years of age and older who are moderately to severely immunocompromised, 2 doses are recommended by NACI, with a 4- to 8-week interval between doses.
      • The reduced dose schedule for unvaccinated individuals 5 years of age and older is applied going forward for those receiving the XBB.1.5-containing COVID-19 vaccine as their first dose.
      • If the primary series was started with non-XBB1.5 COVID-19 vaccine(s) but the series was not completed, those 5 years of age and older who are moderately to severely immunocompromised are recommended to receive a total of 3 doses of COVID-19 vaccines for the primary series (see).
    • For the number of doses and schedules, see Table 2 for those not previously vaccinated and Table 3 for those who started a primary series with a non-XBB.1.5 vaccine.
    • For more information on individuals with moderately to severely immunocompromising conditions, please see the COVID-19 vaccine chapter in the CIG.

Additional considerations for all populations

Table 1. Summary of available XBB.1.5-containing mRNA COVID-19 products by age
Age group Dosage (volume) Description Dilution required Number of doses per vial
Moderna Spikevax XBB.1.5
6 months to under 12 years 25 mcg (0.25 ml)

0.1 mcg / mL

cap color: royal blue;

label color: coral blue

No 10 doses
12 years of age and older 50 mcg (0.5 ml) 5 doses
Pfizer-BioNTech Comirnaty Omicron XBB.1.5
6 months to under 5 years 3 mcg (0.2 ml) Cap and label colour: maroon Yes (with 2.2 mL sterile 0.9% Sodium Chloride) 10 doses after dilution
5 years to under 12 years 10 mcg (0.3 ml) Cap and label colour: blue No 6 doses
12 years of age and older 30 mcg (0.3 ml) Cap and label colour: grey No 6 doses

See product monographs for Moderna Spikevax XBB.1.5 (PDF) and Pfizer-BioNTech Comirnaty Omicron XBB.1.5 (PDF) for storage and handling requirements.

Table 2. Immunization schedule for previously unvaccinated individuals by age starting their vaccinations with XBB.1.5-containing mRNA COVID-19 vaccines
Age group Immunization scheduleFootnote a Products Recommended intervalFootnote b
Schedule for those not moderately or severely immunocompromised
6 months to under 5 years of age
  • 2-dose (Moderna Spikevax) or
  • 3-dose (Pfizer-BioNTech Comirnaty)
  • 25 mcg Moderna Spikevax
  • 3 mcg Pfizer-BioNTech Comirnaty
8 weeks
5 years of age and older
  • 1-dose

Moderna Spikevax

  • 25 mcg (5 to less than 12 years)
  • 50 mcg (12 years of age and older)

Pfizer-BioNTech Comirnaty

  • 10 mcg (5 to less than 12 years)
  • 30 mcg (12 years of age and older)
Not applicable
Schedule for individuals who are moderately to severely immunocompromised
6 months to under 5 years of age
  • 3-dose (Moderna Spikevax)Footnote c or
  • 4-dose (Pfizer-BioNTech Comirnaty)
As above 4 to 8 weeks
5 years of age and older
  • 2-dose
4 to 8 weeks
Footnote a

See "Considerations" and Table 3 regarding individuals who started but did not complete a primary series with a vaccine that was not an XBB.1.5 formulation.

Return to footnote a referrer

Footnote b

For individuals with recent SARS-CoV-2 infection, these are also the suggested intervals between SARS-COV-2 infection and COVID-19 vaccination (see "Additional considerations" section).

Return to footnote b referrer

Footnote c

Moderna Spikevax is the preferred product in children 6 months to under 5 years of age who are moderately to severely immunocompromised due to acceptability and feasibility considerations of only requiring 3 doses instead of 4 doses for Pfizer-BioNTech Comirnaty.

Return to footnote c referrer

Table 3. Summary of number of recommended XBB.1.5-containing mRNA COVID-19 vaccine doses based on previous non-XBB.1.5 vaccination history (i.e., previously received original monovalent or original+BA.1 bivalent or original+BA.4/5 bivalent vaccines [non-XBB.1.5])Footnote a
Age Previous vaccination with only non-XBB.1.5-containing mRNA COVID-19 vaccines Number of doses and interval of XBB.1.5-containing mRNA COVID-19 vaccines to be administered
Moderna Spikevax XBB.1.5 scheduleFootnote b Pfizer-BioNTech Comirnaty Omicron XBB.1.5 scheduleFootnote c
For those not moderately to severely immunocompromised
6 months to under 5 yearsFootnote d 3 or more doses See 2 or more doses

1 dose  

6 months from last dose

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

2 or more doses

1 dose

6 months from last dose

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

See 2 doses or 3 or more doses, as applicable
2 doses See 2 or more doses

1 dose

8 weeks from last dose

1 dose

1 dose

8 weeks from last dose

2 doses

8 weeks from last dose and between doses

5 years of age and older 2 or more doses

1 dose

6 months from last dose

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

1 dose

6 months from last dose

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

1 dose

1 dose

8 weeks from last dose

1 dose

8 weeks from last dose

For those who are moderately to severely immunocompromised
6 months to under 5 yearsFootnote e 4 or more doses See 3 or more doses

1 doseFootnote d

6 months from last dose

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

3 or more doses

1 dose

6 months from last dose

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

See 3 doses or 4 or more doses, as applicable
3 doses See 3 or more doses

1 dose

 

4 to 8 weeks from last dose

2 doses

1 dose

4 to 8 weeks from last dose

Moderna is preferredFootnote e

2 doses

4 to 8 weeks from last dose and between doses

 

1 dose

2 doses

4 to 8 weeks from last dose and between doses

Moderna is preferredFootnote e

3 doses

4 to 8 weeks from last dose and between doses

 

5 years of age and olderFootnote f 3 or more doses

1 dose

6 months from last dose

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

1 dose

6 months from last doseFootnote g

Shorter intervals (i.e., 3 months to less than 6 months) have also not been shown to pose a safety risk

2 doses

1 dose

4 to 8 weeks from last dose

1 doseFootnote g

4 to 8 weeks from last dose

1 dose

2 doses

4 to 8 weeks from last dose and between doses

2 doses

4 to 8 weeks from last dose and between doses

Footnote a

Further details on the recommendations on the use of the XBB.1.5-containing COVID-19 vaccines in previously vaccinated individuals are available in the NACI Guidance on the use of COVID-19 vaccines in the fall of 2023 and the subsequent Addendum to the guidance on the use of COVID-19 vaccines in the fall of 2023.

Return to footnote a referrer

Footnote b

Note for the primary series for children 6 months to under 5 years of age: Follow this column if all past vaccine doses have been Moderna Spikevax vaccines and now also administering Moderna Spikevax XBB.1.5.

Return to footnote b referrer

Footnote c

Note for the primary series for children 6 months to under 5 years of age: Follow this column if now administering Pfizer-BioNTech Comirnaty Omicron XBB.1.5, or if one or more past vaccine doses have been Pfizer-BioNTech Comirnaty vaccines (whether giving Pfizer-BioNTech Comirnaty Omicron XBB.1.5 or Moderna Spikevax XBB.1.5 now).

Return to footnote c referrer

Footnote d

Children 6 months to under 5 years of age who are at high risk for severe illness due to COVID-19 should be vaccinated against COVID-19 and other children in this age group may be vaccinated.

Return to footnote d referrer

Footnote e

For those 6 months to under 5 years of age who are moderately to severely immunocompromised, Moderna Spikevax is preferred because it requires only 3 doses, while Pfizer-BioNTech Comirnaty requires 4 doses.

Return to footnote e referrer

Footnote f

If the primary series was started with non-XBB1.5 COVID-19 vaccine(s), those 5 years of age and older who are moderately to severely immunocompromised are recommended to receive a total of 3 doses of COVID-19 vaccines for the primary series.

Return to footnote f referrer

Footnote g

Children who are moderately to severely immunocompromised and started their primary series with 2 or 3 doses of a non-XBB.1.5 Pfizer-BioNTech product (not the preferred product) when they were less than 5 years of age and are completing their primary series at 5 years of age or older, are recommended to receive a total of 4 doses of COVID-19 vaccine in their primary series (i.e., an additional dose of XBB.1.5-containing vaccine than what is listed in this cell, at 4 to 8 weeks from the previous dose).

Return to footnote g referrer

Table 4. 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 considered for some populations/individuals in some circumstances. Alternative approaches may be reasonable.

Research priorities

Acknowledgments

This statement was prepared by: E Wong, B Warshawsky, M Salvadori, R Krishnan, N Islam, K Young, M Tunis, R Harrison, S Wilson, and S Deeks, on behalf of NACI.

NACI gratefully acknowledges the contribution of: J Zafack, F Khan, J Daniel, C Mauviel, SH Lim, B Lowe, and the NACI Secretariat.

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, M O'Driscoll, J Papenburg, A Pham-Huy, B Sander, and S Wilson.

Liaison representatives: L Bill / M Nowgesic (Canadian Indigenous Nurses Association), LM Bucci (Canadian Public Health Association), S Buchan (Canadian Association for Immunization Research and Evaluation), E Castillo (Society of Obstetricians and Gynaecologists of Canada), J Comeau (Association of Medical Microbiology and Infectious Disease Canada), M Lavoie (Council of Chief Medical Officers of Health), J MacNeil (Centers for Disease Control and Prevention, United States), D Moore (Canadian Paediatric Society), M Naus (Canadian Immunization Committee), M Osmack (Indigenous Physicians Association of Canada), J Potter (College of Family Physicians of Canada), 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), P Fandja (Marketed Health Products Directorate, Health Canada), M Su (COVID-19 Epidemiology and Surveillance, PHAC), S Ogunnaike-Cooke (CIRID, PHAC), C Pham (Biologic and Radiopharmaceutical Drugs Directorate, Health Canada), M Routledge (National Microbiology Laboratory, PHAC), and T Wong (First Nations and Inuit Health Branch, Indigenous Services Canada).

NACI COVID-19 Vaccine Working Group

Members: S Wilson (Chair), M Adurogbangba, M Andrew, M Baca-Estrada, Y-G Bui, H Decaluwe, P De Wals, V Dubey, S Hosseini-Moghaddam, M Miller, D Moore, S Oliver, and E Twentyman.

PHAC Participants: E Abrams, P Doyon-Plourde, N Forbes, M Hersi, N Islam, SJ Ismail, C Jensen, R Krishnan, SH Lim, R Neves Miranda, J Montroy, R Pless, M Salvadori, A Tuite, MC Tunis, B Warshawsky, E Wong, R Ximenes, K Young, 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 Apr 19;28:58,68. https://doi.org/10.1016/j.vaccine.2010.02.035.

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Footnote 2

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. https://doi.org/10.1016/j.vaccine.2020.05.051.

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Footnote 3

Public Health Agency of Canada (PHAC). COVID-19 epidemiology update: Testing and variants. Data cut-off 2023 Sep 12 [Internet]. Ottawa (ON): Government of Canada; 2023 Sep 17 [cited 2023 Oct 17]. Available from: https://health-infobase.canada.ca/covid-19/testing-variants.html.

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Footnote 4

Seroprevalence in Canada. Data cut-off 2023 Jul 31 [Internet]. Montreal (QC): COVID-19 Immunity Task Force (CITF): COVID-19 Immunity Task Force; 2023 Jul 31 [cited 2023 Oct 16]. Available from: https://www.covid19immunitytaskforce.ca/seroprevalence-in-canada/.

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Footnote 5

Doucette EJ, Gray J, Fonseca K, Charlton C, Kanji JN, Tipples G, et al. A longitudinal seroepidemiology study to evaluate antibody response to SARS-CoV-2 virus infection and vaccination in children in Calgary, Canada from July 2020 to April 2022: Alberta COVID-19 Childhood Cohort (AB3C) Study. PLOS ONE. 2023 Apr 6;18(4):e0284046. https://doi.org/10.1371/journal.pone.0284046.

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Footnote 6

Skowronski DM, Kaweski SE, Irvine MA, Kim S, Chuang ESY, Sabaiduc S, et al. Serial cross-sectional estimation of vaccine-and infection-induced SARS-CoV-2 seroprevalence in British Columbia, Canada. CMAJ. 2022 Dec 05;194(47):E1599-609. https://doi.org/10.1503/cmaj.221335.

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Footnote 7

Quach C. Personal Communication. Séroprévalence SARS-COV-2 sur échantillons résiduels (Oct 27, 2022). 2023 Aug 09.

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Footnote 8

Zinszer K. Personal Communication. Encore study Children and COVID19 Montreal seroprevalence study. Feb – Jun 2023 Round 5 preliminary results. 2023 Aug 10.

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Footnote 9

Ahira S. Personal Communication. COVID-19 seroepidemiology in Children Using Retrieved POPCORN site Leftover Samples (CURNLS). 2023 Aug 10.

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Footnote 10

Bobrovitz N, Ware H, Ma X, Li Z, Hosseini R, Cao C, et al. Protective effectiveness of previous SARS-CoV-2 infection and hybrid immunity against the omicron variant and severe disease: A systematic review and meta-regression. Lancet Infect Dis. 2023 May;23(5). https://doi.org/10.1016/S1473-3099(22)00801-5.

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Footnote 11

Carazo S, Skowronski DM, Brisson M, Barkati S, Sauvageau C, Brousseau N, et al. Protection against omicron (B.1.1.529) BA.2 reinfection conferred by primary omicron BA.1 or pre-omicron SARS-CoV-2 infection among health-care workers with and without mRNA vaccination: a test-negative case-control study. Lancet Infect Dis. 2023 Jan;23(1):45-55. https://doi.org/10.1016/S1473-3099(22)00578-3.

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Footnote 12

Carazo S, Skowronski DM, Brisson M, Sauvageau C, Brousseau N, Fafard J, et al. Prior infection- and/or vaccine-induced protection against Omicron BA.1, BA.2 and BA.4/BA.5-related hospitalisations in older adults: a test-negative case-control study in Quebec, Canada. medRxiv. 2022 Dec 27. https://doi.org/10.1101/2022.12.21.22283740.

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Footnote 13

Altarawneh HN, Chemaitelly H, Ayoub HH, Tang P, Hasan MR, Yassine HM, et al. Effects of Previous Infection and Vaccination on Symptomatic Omicron Infections. N Engl J Med. 2022 Jul 7;387(1):21-34. https://doi.org/10.1056/NEJMoa2203965.

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Footnote 14

Cerqueira-Silva T, de Araujo Oliveira V, Paixão ES, Florentino PTV, Penna GO, Pearce N, et al. Vaccination plus previous infection: protection during the omicron wave in Brazil. Lancet Infect Dis. 2022 May 16. https://doi.org/10.1016/S1473-3099(22)00288-2.

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Footnote 15

Chin ET, Leidner D, Lamson L, Lucas K, Studdert DM, Goldhaber-Fiebert JD, et al. Protection against Omicron from Vaccination and Previous Infection in a Prison System. N Engl J Med. 2022 Nov 10;387(19):1770-82. https://doi.org/10.1056/NEJMoa2207082.

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Footnote 16

Vicentini M, Venturelli F, Mancuso P, Bisaccia E, Zerbini A, Massari M, et al. Risk of SARS-CoV-2 Reinfection by Vaccination Status, Predominant Variant, and Time from Previous Infection: A Cohort Study in Italy. SSRN. 2022 Jun 09. https://doi.org/10.2139/ssrn.4132329.

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Footnote 17

Lind ML, Robertson AJ, Silva J, Warner F, Coppi AC, Price N, et al. Effectiveness of Primary and Booster COVID-19 mRNA Vaccination against Omicron Variant SARS-CoV-2 Infection in People with a Prior SARS-CoV-2 Infection. medRxiv. 2022 Apr 25. https://doi.org/10.1101/2022.04.19.22274056.

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Footnote 18

Spreco A, Dahlström Ö, Jöud A, Nordvall D, Fagerström C, Blomqvist E, et al. Effectiveness of the BNT162b2 mRNA Vaccine Compared with Hybrid Immunity in Populations Prioritized and Non-Prioritized for COVID-19 Vaccination in 2021-2022: A Naturalistic Case-Control Study in Sweden. Vaccines (Basel). 2022 Aug 7;10(8):1273. https://doi.org/10.3390/vaccines10081273.

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Footnote 19

Moayyedi P. The effects of vaccination in immunocompromised people: Systematic review of research studies on immunogenicity, safety, and efficacy/effectiveness of COVID-19 vaccines in immunocompromised individuals [Internet]. Toronto (ON): SPOR Evidence Alliance; 2021 Aug 25 [cited 2023 Oct 16]. Available from: https://sporevidencealliance.ca/wp-content/uploads/2023/05/SPOREA-COVIDEND_Immunocompromised_Final-Report_2021.08.25.pdf.

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Footnote 20

Moayyedi P. The effects of third and fourth dose vaccination in immunocompromised people: Systematic review of research studies on immunogenicity, safety, and efficacy/effectiveness of third and fourth dose COVID-19 vaccines in immunocompromised individuals [Internet]. Toronto (ON): SPOR Evidence Alliance; 2022 Apr 13 [cited 2023 Oct 16]. Available from: https://sporevidencealliance.ca/wp-content/uploads/2023/05/The-effects-of-third-and-fourth-dose-vaccination-in-immunocompromised-people.pdf.

Return to footnote 20 referrer

Footnote 21

Linkins L. Rapid Evidence Synthesis: COVID-19 Vaccine Effectiveness in Unvaccinated Moderate to Severely Immunocompromised People with a Previous Infection. McMaster Health Forum. 2023 Sep 14. Available from: https://www.mcmasterforum.org/docs/default-source/product-documents/rapid-responses/covid-19-vaccine-effectiveness-in-unvaccinated-moderate-to-severely-immunocompromised-people-with-a-previous-infection.pdf.

Return to footnote 21 referrer

Footnote 22

Kavikondala S, Haeussler K, Wang X, Spellman A, Bausch-Jurken MT, Sharma P, et al. Immunogenicity of mRNA-1273 and BNT162b2 in Immunocompromised Patients: Systematic Review and Meta-Analysis Using GRADE. medRxiv. 2023 Aug 13. https://doi.org/10.1101/2023.08.09.23293898.

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Footnote 23

Boehmer TK, Kompaniyets L, Lavery AM, Hsu J, Ko JY, Yusuf H, et al. Association between COVID-19 and myocarditis using hospital-based administrative data - United States, March 2020-January 2021. MMWR Morb Mortal Wkly Rep. 2021 Sep 3;70(35):1228,1232. doi: 10.15585/mmwr.mm7035e5. https://doi.org/10.15585/mmwr.mm7035e5.

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Footnote 24

Public Health Agency of Canada (PHAC). COVID-19 vaccination: Doses administered. Data cut-off 2023 Sep 10 [Internet]. Ottawa (ON): Government of Canada; 2023 Sep 15 [cited 2023 Oct 17]. Available from: https://health-infobase.canada.ca/covid-19/vaccine-administration/.

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Footnote 25

Public Health Agency of Canada (PHAC). Archived 45: Updated guidance on COVID-19 vaccine booster doses in Canada [Internet]. Ottawa (ON): Public Health Agency of Canada (PHAC): Government of Canada; 2022 Oct 21. Post-market safety of original mRNA booster doses [cited 2023 Oct 18]. Available from: https://www.canada.ca/en/public-health/services/immunization/national-advisory-committee-on-immunization-naci/guidance-covid-19-vaccine-booster-doses.html#a4.7.

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Footnote 26

Buchan SA, Seo CY, Johnson C, Alley S, Kwong JC, Nasreen S, et al. Epidemiology of Myocarditis and Pericarditis Following mRNA Vaccination by Vaccine Product, Schedule, and Interdose Interval Among Adolescents and Adults in Ontario, Canada. JAMA Netw Open. 2022 Jun 01;5(6):e2218505. https://doi.org/10.1001/jamanetworkopen.2022.18505.

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Footnote 27

Public Health Agency of Canada (PHAC). COVID-19 vaccination: Vaccination coverage in Canada. Data cut-off 2023 Sep 10 [Internet]. Ottawa (ON): Government of Canada; 2023 Sep 15 [cited 2023 Oct 23]. Available from: https://health-infobase.canada.ca/covid-19/vaccination-coverage/.

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