Relative Effectiveness of MF59 Adjuvanted Trivalent Influenza Vaccine vs Nonadjuvanted Vaccines During the 2019–2020 Influenza Season

BACKGROUND: Age-related immunosenescence may impair the immune response to vaccination in older adults. Adjuvanted influenza vaccines are designed to overcome immune senescence in older adults. This study estimated the relative vaccine effectiveness (rVE) of MF59-adjuvanted trivalent inactivated influenza vaccine (aIIV3) vs egg-derived quadrivalent inactivated influenza vaccine (IIV4e) and high-dose trivalent inactivated influenza vaccine (HD-IIV3) in preventing influenza-related medical encounters in the 2019–2020 US season. METHODS: This retrospective cohort study used electronic medical records linked to pharmacy and medical claims data. The study population included adults age ≥65 years with a record of aIIV3, IIV4e, or HD-IIV3 vaccination. A doubly robust inverse probability of treatment weighting model was used to derive adjusted odds ratios (ORs). rVE was calculated by (1 – OR(adjusted))*100 and was determined overall and separately for age subgroups. An exploratory analysis evaluated the outcome separately in inpatient and outpatient settings. RESULTS: Subjects received aIIV3 (n = 936 508), IIV3e (n = 651 034), and HD-IIV3 (n = 1 813 819), and influenza-related medical encounters were recorded in 0.5%, 0.9%, and 0.7% of each cohort, respectively. Overall, the rVE of aIIV3 was 27.5% (95% CI, 24.4% to 30.5%) vs IIV4e and 13.9% (95% CI, 10.7% to 17.0%) vs HD-IIV3. aIIV3 had a more favorable rVE in inpatient and outpatient settings. Findings remained consistent across age subgroups and during alternative seasonal dates. CONCLUSIONS: Adults age ≥65 years vaccinated with aIIV3 had fewer influenza-related medical encounters compared with IIV4e or HD-IIV3 during the 2019–2020 US influenza season.