BACKGROUND
Following historically low influenza activity during the 2020-2021 season, the United States saw an increase in influenza circulating during the 2021-2022 season. Most viruses belonged to the influenza A(H3N2) 3C.2a1b 2a.2 subclade.
METHODS
We conducted a test-negative case-control analysis among adults ≥18 years of age at three sites within the VISION Network. Encounters included emergency department/urgent care (ED/UC) visits or hospitalizations with ≥1 acute respiratory illness (ARI) discharge diagnosis codes and molecular testing for influenza. Vaccine effectiveness (VE) was calculated by comparing the odds of influenza vaccination ≥14 days before the encounter date between influenza-positive cases (type A) and influenza-negative and SARS-CoV-2-negative controls, applying inverse probability-to-be-vaccinated weights, and adjusting for confounders.
RESULTS
86,732 ED/UC ARI-associated encounters (7,696 [9%] cases) and 16,805 hospitalized ARI-associated encounters (649 [4%] cases) were included. VE against influenza-associated ED/UC encounters was 25% (95% confidence interval (CI): 20-29%) and 25% (95%CI: 11-37%) against influenza-associated hospitalizations. VE against ED/UC encounters was lower in adults ≥65 years of age (7%; CI: -5-17%) or with immunocompromising conditions (4%, CI:-45-36%).
CONCLUSIONS
During an influenza A(H3N2)-predominant influenza season, modest VE was observed. These findings highlight the need for improved vaccines, particularly for A(H3N2) viruses that are historically associated with lower VE.