Influenza Vaccine Effectiveness Against Influenza A(H3N2)-Related Illness in the United States During the 2021-2022 Influenza Season

Background. In the United States, influenza activity during the 2021-2022 season was modest and sufficient enough to estimate influenza vaccine effectiveness for the first time since the beginning of the COVID-19 pandemic. We estimated influenza vaccine effectiveness against lab-confirmed outpatient acute illness caused by predominant A(H3N2) viruses. Methods. Between October 2021 and April 2022, research staff across 7 sites enrolled patients aged [≥]6 months seeking outpatient care for acute respiratory illness with cough. Using a test-negative design, we assessed VE against influenza A(H3N2). Due to strong correlation between influenza and SARS-CoV-2 vaccination, participants who tested positive for SARS-CoV-2 were excluded from vaccine effectiveness estimations. Estimates were adjusted for site, age, month of illness, race/ethnicity and general health status. Results. Among 6,260 participants, 468 (7%) tested positive for influenza only, including 440 (94%) for A(H3N2). All 206 sequenced A(H3N2) viruses were characterized as belonging to genetic group 3C.2a1b subclade 2a.2, which has antigenic differences from the 2021-2022 season A(H3N2) vaccine component that belongs to clade 3C.2a1b subclade 2a.1. After excluding 1,948 SARS-CoV-2 positive patients, 4,312 patients were included in analyses of influenza VE; 2,463 (57%) were vaccinated against influenza. Effectiveness against A(H3N2) for all ages was 36% (95%CI, 20-49%) overall; 40% (95%CI, 24-53%) for those aged 6 months-49 years; and 10% (95%CI, -60-49%) for those aged [≥]50 years. Conclusion. Influenza vaccination in 2021-2022 provided protection against influenza A(H3N2)-related outpatient visits among young persons, with no measurable protection among older adults.