Comparative Effectiveness of Licensed Influenza Vaccines in Preventing Influenza-related Medical Encounters and Hospitalizations in the 2022-2023 Influenza Season Among Adults ≥65 Years of Age

Background: Influenza causes substantial morbidity, particularly among older individuals. Updated data on the effectiveness of currently licensed vaccines in this population are needed.

Methods: At Kaiser Permanente Southern California, we conducted a retrospective cohort study to evaluate comparative vaccine effectiveness (cVE) of high-dose (HD), adjuvanted, and standard-dose (SD) cell-based influenza vaccines, relative to the SD egg-based vaccine. We included adults aged ≥65 years who received an influenza vaccine between 1 August 2022 and 31 December 2022, with follow-up up to 20 May 2023. Primary outcomes were: (1) influenza-related medical encounters and (2) polymerase chain reaction (PCR)-confirmed influenza-related hospitalization. Adjusted hazard ratios (aHR) were estimated by Cox proportional hazards regression, adjusting for confounders using inverse probability of treatment weighting (IPTW). cVE (%) was calculated as (1-aHR) × 100 when aHR ≤1, and ([1/aHR]-1) × 100 when aHR >1.

Results: Our study population (n = 495 119) was 54.9% female, 46.3% non-Hispanic White, with a median age of 73 years (interquartile range [IQR] 69-79). Characteristics of all groups were well balanced after IPTW. Adjusted cVEs against influenza-related medical encounters in the HD, adjuvanted, and SD cell-based vaccine groups were 9.1% (95% confidence interval [CI]: .9, 16.7), 16.9% (95% CI: 1.7, 29.8), and -6.3 (95% CI: -18.3, 6.9), respectively. Adjusted cVEs against PCR-confirmed hospitalization in the HD, adjuvanted, and SD cell-based groups were 25.1% (95% CI: .2, 43.8), 61.6% (95% CI: 18.1, 82.0), and 26.4% (95% CI: -18.3, 55.7), respectively.

Conclusions: Compared to the SD egg-based vaccine, HD and adjuvanted vaccines conferred additional protection against influenza-related outcomes in the 2022-2023 season in adults ≥65 years. Our results provide real-world evidence of the comparative effectiveness of currently licensed vaccines.