P-63. Real-World Effectiveness of <i>Influenza</i> Vaccine Over a Decade During the 2011–2021 Seasons: Implications of Vaccine Mismatch

Abstract Background Influenza imposes a significant healthcare burden in Korea, leading the government to initiate a national immunization program. Previous studies on vaccine effectiveness (VE) were limited to single-season estimation in Korea. Matching status of epidemic influenza virus strains and vaccine strains in each season from the 2011/2012 to 2020/2021 seasons Methods This multicenter prospective cohort study enrolled patients with influenza-like illnesses at 10 medical centers in Korea from 2011–2021. The demographic and clinical data were collected from questionnaire surveys and electronic medical records. Using a test-negative design, we aimed to investigate the effectiveness of a seasonal influenza vaccine for antigenic matching of the vaccine and circulating viral strains over 10 seasons. Vaccine effectiveness against laboratory-confirmed influenza Results Overall, 5,322 adults aged ≥65 years were enrolled. Only three (33.3%) of nine seasons showed &amp;gt;70% antigenic match between vaccine and circulating strains. Influenza VE was significantly variable by season, ranging from -46.9% (95%confidence interval [CI]: -127.6–5.2) in the 2011/12 season to 47.7% (95%CI: 22.6–64.7) in the 2016/17 season. A significant difference was observed in the VE depending on whether the vaccine strains matched with epidemic strains: 28.8% (95%CI: 8.8–44.8) in matched seasons versus -12.0% (95%CI: -30.0–3.7) in mismatched seasons. Across the study period, influenza-related hospitalizations were reduced by 13.6% (95%CI: 0.7–24.8) with vaccination. In a subgroup analysis, the VE against influenza-related hospitalization was 48.4% (95%CI 29.6–62.2) in A/H3N2 dominant seasons and 53.8% (95%CI: -73.4–87.7) in A/H1N1 dominant seasons, respectively. Vaccine effectiveness against laboratory-confirmed influenza by the matching status of vaccine and epidemic strains Conclusion Influenza vaccine mismatch was frequent over the study period, leading to negligibly low VE in mismatched seasons. Influenza vaccination reduces the risk of influenza-related hospitalizations. Estimates of vaccine effectiveness for influenza-related hospitalizations Disclosures All Authors: No reported disclosures