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. 2022 Oct 1;48(10):473-483.

National Influenza Annual Report, Canada, 2021-2022: A brief, late influenza epidemic

Steven Buckrell  1 , Myriam Ben Moussa  1 , Tammy Bui  1 , Abbas Rahal  1 , Kara Schmidt  1 , Liza Lee  1 , Nathalie Bastien  2 , Christina Bancej  1
Affiliations

National Influenza Annual Report, Canada, 2021-2022: A brief, late influenza epidemic

Steven Buckrell et al. Can Commun Dis Rep. .

Abstract

Canadian seasonal influenza circulation had been suppressed since the beginning of the coronavirus disease 2019 (COVID-19) pandemic. This suppression was reported globally and generated concern that the return of community influenza circulation could be intense and that co-circulation of influenza and severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) was possible and potentially severe. Community circulation of influenza returned to Canada during the 2021-2022 influenza season. The influenza epidemic began in week 16 (mid-April 2022) and lasted only nine weeks. This epidemic was driven by influenza A(H3N2) and was exceptionally late in the season, low in intensity and short in length. Community co-circulation of influenza and SARS-CoV-2 was observed in Canada for the first time during the 2021-2022 seasonal influenza epidemic. The unusual characteristics of the 2021-2022 influenza epidemic suggest that a breadth of factors moderate transmission dynamics of the two viruses. Concerns of an intense seasonal influenza epidemic did not come to fruition during the 2021-2022 season; therefore, high influenza susceptibility remains, as does predisposition to larger influenza epidemics. Ongoing circulation of SARS-CoV-2 creates uncertainty about dynamics of future influenza epidemics, but influenza vaccination remains a key public health intervention available to protect Canadians. Public health authorities need to remain vigilant, maintain surveillance and continue to plan for both heightened seasonal influenza circulation and for the potential for endemic co-circulation of influenza and SARS-CoV-2.

Keywords: COVID-19; influenza; influenza-like illness; pandemic preparedness; surveillance.

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Conflict of interest statement

Competing interests None.

Figures

Figure 1
Figure 1
Percentage of influenza tests positive in Canada by surveillance weeka,b a Comparison of 2021–2022 influenza season to previous seasons, 2014–2015 to 2019–2020 and 2020–2021 b The shaded area represents the maximum and minimum percentage of tests positive reported by week from 2014–2015 to 2019–2020. The epidemic threshold is 5% tests positive for influenza. When it is exceeded, and a minimum of 15 weekly influenza detections is reported, a seasonal influenza epidemic is declared
Figure 2
Figure 2
Number of positive influenza tests and percentage of tests positive in Canada, by type, subtype and surveillance week for the 2021–2022 influenza seasona a The shaded area indicates weeks where the positivity rate was at least 5% and a minimum of 15 positive tests were observed, representing the 2021–2022 seasonal influenza epidemic
Figure 3
Figure 3
Number of influenza surveillance regions reporting sporadic or localized activity by week in Canada, 2021–2022 influenza season
Figure 4
Figure 4
Percentage of visits for ILI reported by sentinel primary care providers in Canada by surveillance weeka,b Abbreviation: ILI, influenza-like illness a Comparison of 2021–2022 influenza season to previous seasons, 2014–2015 to 2019–2020 and 2020–2021 b The shaded area represents the maximum and minimum percentage of visits for ILI reported by week from 2014–2015 to 2019–2020
Figure 5
Figure 5
Percentage of FluWatcher participants reporting cough and fever in Canada by surveillance weeka,b a Comparison of 2021–2022 influenza season to previous seasons, 2016–2017 to 2019–2020 and 2020–2021 b The shaded area represents the maximum and minimum percentage of percentage of participants reporting cough and fever by week, from 2016–2017 to 2019–2020
Figure 6
Figure 6
Percentage of influenza and SARS-CoV-2 laboratory tests positive and percentage of FluWatchers reporting cough and fever in Canada by surveillance week, 2021–2022 influenza season Abbreviation: SARS-CoV-2, severe acute respiratory syndrome coronavirus 2
Figure 7
Figure 7
Preliminary number of influenza-associated paediatric hospitalizations reported by IMPACT, by week of admissiona,b Abbreviation: IMPACT, Canadian Immunization Monitoring Program ACTive a Comparison of 2021–2022 influenza season to previous seasons, 2014–2015 to 2019–2020 b The shaded area represents the maximum and minimum hospitalizations reported by week of admission, from 2014–2015 to 2019–2020
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