Vaccine Effectiveness Against Influenza-Associated Urgent Care, Emergency Department, and Hospital Encounters During the 2021–2022 Season, VISION Network

Abstract

Abstract 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 3 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 severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)–negative controls, applying inverse probability-to-be-vaccinated weights, and adjusting for confounders. Results In total, 86 732 ED/UC ARI-associated encounters (7696 [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%; 95% CI, −5% to 17%) or with immunocompromising conditions (4%; 95% CI, −45% to 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.