Relative Effectiveness of Adjuvanted Trivalent Inactivated Influenza Vaccine Versus Egg-derived Quadrivalent Inactivated Influenza Vaccines and High-dose Trivalent Influenza Vaccine in Preventing Influenza-related Medical Encounters in US Adults ≥ 65 Years During the 2017-2018 and 2018-2019 Influenza Seasons

Background. The effectiveness of standard, egg-derived quadrivalent influenza vaccines (IIV4) may be reduced in adults >= 65 years of age, largely because of immunosenescence. An MF59-adjuvanted trivalent influenza vaccine (aIIV3) and a high-dose trivalent influenza vaccine (HD-IIV3) offer older adults enhanced protection versus standard vaccines. This study compared the relative effectiveness of aIIV3 with IIV4 and HD-IIV3 in preventing influenza-related medical encounters over 2 US influenza seasons. Methods. This retrospective cohort study included US patients >= 65 years vaccinated with aIIV3, IIV4, or HD-IIV3. The outcome of interest was the occurrence of influenza-related medical encounters. Data were derived from a large dataset comprising 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. Relative vaccine effectiveness (rVE) was determined using the formula (% VE = 1 - ORadjusted) x 100. Results. In 2017-2018, cohorts included: aIIV3, n = 524 223; IIV4, n = 917 609; and HD-IIV3, n = 3 377 860. After adjustment, 2017-2018 rVE of aIIV3 versus IIV4 was 18.2 (95% confidence interval [CI], 15.8-20.5); aIIV3 vs. HD-IIV3 was 7.7 (95% CI, 2.3-12.8). In 2018-2019, cohorts included: aIIV3, n = 1 031 145; IIV4, n = 915 380; HD-IIV3, n = 3 809 601, with adjusted rVEs of aIIV3 versus IIV4 of 27.8 (95% CI, 25.7-29.9) and vs. HD-IIV3 of 6.9 (95% CI, 3.1-10.6). Conclusion. In the 2017-2018 and 2018-2019 influenza seasons in the United States, aIIV3 demonstrated greater reduction in influenza-related medical encounters than IIV4 and HD-IIV3 in adults >= 65 years.