Interim Estimates of 2021–22 Seasonal Influenza Vaccine Effectiveness — United States, February 2022

A late sharp increase in influenza detections and low interim vaccine effectiveness against the circulating A(H3N2) strain, Denmark, 2021/22 influenza season up to 25 March 2022

“…In Denmark, almost all COVID-19 interventions and restrictions were lifted by the end of January 2022 (end of week 4), which might explain the sharp increase in the influenza transmission from week 6 in 2022 and onwards. The low to no effect of the administered vaccines in both hospitalised and non-hospitalised patients aged 7 years and above against the circulating influenza A stain is similar to findings in a study from the United States where the overall VE against medically attended outpatients with influenza A(H3N2) virus was estimated to 16% (95% CI: −16 to 39) [ 5 ]. In Denmark, the majority of the influenza vaccines were administered during October 2021, which was almost 5 months before the influenza A(H3N2)-dominated increase was observed.…”

Influenza returns with a season dominated by clade 3C.2a1b.2a.2 A(H3N2) viruses, WHO European Region, 2021/22

“…For the 2021/22 influenza season up to week 10, A(H3) viruses have dominated in the Region and the majority (98%) of characterised viruses up to week 4 2022 fell in genetic clade 3C.2a1b.2a.2, being distinct from the NH vaccine component for the 2021/22 season vaccines. This may have negative implications for the vaccine effectiveness, as has been described by the United States interim estimates, whereby seasonal influenza vaccination did not reduce the risk for outpatient respiratory illness caused by influenza A(H3N2) viruses that have predominated up to mid-February 2022 [ 12 ]. Of the few A(H1N1)pdm09 and B/Victoria-lineage viruses detected, most have fallen in clades 6B.1A.5a.1 and V1A.3a.2, respectively, and show reduced recognition by post-infection ferret antisera induced by the 2021/22 vaccine component viruses, which were clade 6B.1A.5a.2 and V1A.3 viruses.…”

Antigenic and virological properties of an H3N2 variant that continues to dominate the 2021–22 Northern Hemisphere influenza season

“…Vaccine effectiveness against 2a2 was found to be low during an outbreak on a college campus in the United States during November 2021 ( Delahoy et al., 2021 ). Interim estimates suggest that the 2021–22 influenza vaccine did not reduce the risk for outpatient respiratory illness caused by H3N2 viruses in the United States ( Chung et al., 2022 ). It is therefore important to understand the virological and antigenic properties of 2a2 H3N2 viruses.…”