Rapid communication Open Access
Like 0


Influenza virus circulation virtually ceased in Canada during the COVID-19 pandemic, re-emerging with the relaxation of restrictions in spring 2022. Using a test-negative design, the Canadian Sentinel Practitioner Surveillance Network reports 2021/22 vaccine effectiveness of 36% (95% CI: −38 to 71) against late-season illness due to influenza A(H3N2) clade 3C.2a1b.2a.2 viruses, considered antigenically distinct from the 3C.2a1b.2a.1 vaccine strain. Findings reinforce the World Health Organization’s decision to update the 2022/23 northern hemisphere vaccine to a more representative A(H3N2) clade 3C.2a1b.2a.2 strain.


Article metrics loading...

Loading full text...

Full text loading...



  1. Groves HE, Piché-Renaud PP, Peci A, Farrar DS, Buckrell S, Bancej C, et al. The impact of the COVID-19 pandemic on influenza, respiratory syncytial virus, and other seasonal respiratory virus circulation in Canada: A population-based study. Lancet Reg Health Am. 2021;1:100015.  https://doi.org/10.1016/j.lana.2021.100015  PMID: 34386788 
  2. Public Health Agency of Canada (PHAC). Influenza weekly reports 2021-22 season. Ottawa: PHAC; 2022. [Accessed 6 September 2022]. Available from: https://www.canada.ca/content/dam/phac-aspc/documents/services/publications/diseases-conditions/fluwatch/2021-2022/weeks-30-34-july-24-august-27-2022/fluwatch-weeks-30-34.pdf
  3. Doll MK, Pettigrew SM, Ma J, Verma A. Effects of confounding bias in coronavirus disease 2019 (COVID-19) and influenza vaccine effectiveness test-negative designs due to correlated influenza and COVID-19 vaccination behaviors. Clin Infect Dis. 2022;75(1):e564-71.  https://doi.org/10.1093/cid/ciac234  PMID: 35325923 
  4. Canadian Influenza Sentinel Practitioner Surveillance Network (SPSN). Influenza vaccine effectiveness estimates % (95% CI), 2004-05 to 2018-19 seasons. Vancouver: British Columbia Centre for Disease Control. [Accessed: 6 September 2022]. Available from: http://www.bccdc.ca/resource-gallery/Documents/Statistics%20and%20Research/Publications/Epid/Influenza%20and%20Respiratory/SPSN_VE_By_Year_Table.pdf
  5. World Health Organization (WHO). Recommendations for influenza vaccine composition; 2022. [Accessed 18 September 2022]. Available from: https://www.who.int/teams/global-influenza-programme/vaccines/who-recommendations
  6. Firth D. Bias reduction of maximum likelihood estimates. Biometrika. 1993;80(1):27-38.  https://doi.org/10.1093/biomet/80.1.27 
  7. Heinze G, Schemper M. A solution to the problem of separation in logistic regression. Stat Med. 2002;21(16):2409-19.  https://doi.org/10.1002/sim.1047  PMID: 12210625 
  8. Devika S, Jeyaseelan L, Sebastian G. Analysis of sparse data in logistic regression in medical research: A newer approach. J Postgrad Med. 2016;62(1):26-31.  https://doi.org/10.4103/0022-3859.173193  PMID: 26732193 
  9. Bolton MJ, Ort JT, McBride R, Swanson NJ, Wilson J, Awofolaju M, et al. Antigenic and virological properties of an H3N2 variant that continues to dominate the 2021-22 Northern Hemisphere influenza season. Cell Rep. 2022;39(9):110897.  https://doi.org/10.1016/j.celrep.2022.110897  PMID: 35649381 
  10. Centers for Disease Control and Prevention (CDC). Interim US flu vaccine effectiveness (VE) data for 2021-2022. Atlanta: CDC. [Accessed 6 September 2022]. Available from: https://www.cdc.gov/flu/vaccines-work/2021-2022.html
  11. Santé publique France. Grippe. Bilan préliminaire de l’épidémie de grippe 2021-22. [Influenza. Preliminary assessment of the 2021/22 influenza epidemic. Saint-Maurice: Santé publique France; 2022. French. Available from: https://www.santepubliquefrance.fr/content/download/440665/document_file/Bulletin_grippe_bilan_21-22.pdf
  12. Skowronski DM, Sabaiduc S, Leir S, Rose C, Zou M, Murti M, et al. Paradoxical clade- and age-specific vaccine effectiveness during the 2018/19 influenza A(H3N2) epidemic in Canada: potential imprint-regulated effect of vaccine (I-REV). Euro Surveill. 2019;24(46):1900585.  https://doi.org/10.2807/1560-7917.ES.2019.24.46.1900585  PMID: 31771709 
  13. Flannery B, Kondor RJG, Chung JR, Gaglani M, Reis M, Zimmerman RK, et al. Spread of antigenically drifted influenza A(H3N2) viruses and vaccine effectiveness in the United States during the 2018-2019 season. J Infect Dis. 2020;221(1):8-15.  https://doi.org/10.1093/infdis/jiz543  PMID: 31665373 
  14. Kissling E, Pozo F, Buda S, Vilcu A-M, Gherasim A, Brytting M, et al. Low 2018/19 vaccine effectiveness against influenza A(H3N2) among 15-64-year-olds in Europe: exploration by birth cohort. Euro Surveill. 2019;24(48):1900604.  https://doi.org/10.2807/1560-7917.ES.2019.24.48.1900604  PMID: 31796152 
  15. Kelvin AA, Zambon M. Influenza imprinting in childhood and the influence on vaccine response later in life. Euro Surveill. 2019;24(48):1900720.  https://doi.org/10.2807/1560-7917.ES.2019.24.48.1900720  PMID: 31796156 

Data & Media loading...

Supplementary data

Submit comment
Comment moderation successfully completed
This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error