MF59®-adjuvanted trivalent inactivated influenza vaccine (aIIV3) and high-dose trivalent inactivated influenza vaccine (HD-IIV3) elicit an enhanced immune response in older adults compared to standard, quadrivalent inactivated influenza vaccines (IIV4). We sought to determine the relative vaccine effectiveness (rVE) of aIIV3 versus IIV4 and HD-IIV3 in preventing influenza-related medical encounters in this retrospective cohort study involving adults ≥65 years with ≥1 health condition during the 2017-2018 and 2018-2019 influenza seasons. Data were obtained from primary and specialty care electronic medical records linked with pharmacy and medical claims. Adjusted odds ratios (OR) were derived from an inverse probability of treatment-weighted sample adjusted for age, sex, race, ethnicity, geographic region, vaccination week, and health status. rVE was determined using the formula (% rVE = 1 - ORadjusted) × 100. Analysis sets included 1,755,420 individuals for the 2017-2018 season and 2,055,012 for the 2018-2019 season. Compared to IIV4, aIIV3 was 7.1% (95% confidence interval 3.3-10.8) and 20.4% (16.2-24.4) more effective at preventing influenza-related medical encounters in the 2017-2018 and 2018-2019 seasons, respectively. Comparable effectiveness was observed with HD-IIV3 across both seasons. Our results support improved effectiveness of aIIV3 vs IIV4 in a vulnerable population of older adults at high risk of influenza and its complications.
Keywords: MF59-adjuvanted influenza vaccine; high-risk; influenza-related medical encounters; older adults; relative vaccine effectiveness.