Unusual Spring Wave of Late Flu in Canada 2022

Shinhye et al. investigating the impact of COVID-19 measures on flu circulation in Canada between 2020 and 2022 and the effectiveness of the vaccine against late season flu illness due to A(H3N2) in 2022.

At the beginning of the COVID-19 pandemic, the circulation of the flu virus in Canada stopped due to the implementation of public health measures against COVID-19, such as physical distancing and wearing masks. In the spring of 2022, when these measures were relaxed, the flu virus resurfaced.

Effectiveness of Late Season Flu Vaccines

VE analyzes were performed using a test negative design1 including nasal or nasopharyngeal samples. The analyzes included test results from patients older than 1 year, within 7 days of acute onset of a flu-like illness at community clinics or COVID-19 sites in the Canadian provinces of Alberta, British Columbia and Ontario.

Of the 327 eligible samples collected during the analysis period, 13% tested positive for influenza A(H3N2). All belonged to the clade 3C.2a1b.2a.2 which is considered antigenically different from the A(H3N2) 3C.2a1b.2a.1 strain in the 2021/22 flu vaccine. However, the influenza A(H3N2) clade found in Canada was similar to that in the vaccine for the upcoming Northern Hemisphere 2022/23 season.

Study results support flu vaccine composition decision 2022/23

Adjusted (age group, province, comorbidity and calendar month) VE against influenza A(H3N2) was 36% (95% confidence interval -38 to 71) in the three Canadian provinces. The authors note that the “confidence intervals around our VE estimates are wide and we cannot rule out an interpretation of no protection. However, point estimates remain the most likely findings.”

In addition, the results are similar to 2021/22 VE estimates against influenza A(H3N2) reported from the United States and Europe [2,3]. Authors also emphasize that “despite the mismatch of the vaccine category 3C.2a1b.2a.1 strain against the circulating clade 3C.2a1b.2a.2 viruses and an unusually long time since vaccination, the Canadian SPSN shows that the vaccine for 2021/ 22 reduced the risk of medically supervised influenza A(H3N2) disease by about a third during the late spring wave.”

Finally, based on their analysis, the authors conclude that “the findings reinforce the World Health Organization’s decision to switch to a clade 3C.2a1b.2a.2 strain for the 2022/23 vaccine in the Northern Hemisphere”.


1 Design used in vaccine efficacy studies with cases testing positive for a specific disease, compared to a control group testing negative in the same facility.

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