Interim Effectiveness of the Influenza Vaccine for the 2024-2025 Respiratory Viral Season

BackgroundThe purpose of this study was to evaluate the effectiveness of the influenza vaccine during the 2024-2025 respiratory viral season. MethodsEmployees of Cleveland Clinic in employment in Ohio on October 1, 2024, were included. The cumulative incidence of influenza among those in the vaccinated and unvaccinated states was compared over the following 33 weeks. Protection provided by vaccination (analyzed as a time- dependent covariate) was evaluated using multivariable Cox proportional hazards regression, including adjusting for influenza activity as an effect modifier. ResultsAmong 53402 employees, 43920 (82.2%) were vaccinated by the end of the study. Influenza occurred in 1130 (2.12%) during the study. The cumulative incidence of influenza was similar for the vaccinated and unvaccinated states early, but over time the cumulative incidence of influenza increased more rapidly among the vaccinated than the unvaccinated. In a multivariable Cox proportional hazards regression analysis, the risk of influenza was not significantly different during periods when influenza activity was low (HR, .70; 95% C.I., .42 - 1.15; P = .15) or medium (HR, 1.20; 95% C.I., .86 - 1.67; P = .28), but was significantly higher for the vaccinated state than the unvaccinated state when influenza activity was high (HR, 1.33; 95% C.I., 1.07 - 1.64; P = .009). ConclusionsThis study was unable to find a protective influence of influenza vaccination among working-aged adults during the 2024-2025 respiratory viral season and found that influenza vaccination was associated with a higher risk of influenza when influenza activity was high. SummaryAmong 53402 working-aged Cleveland Clinic employees, we were unable to find a protective influence of influenza vaccination during the 2024-2025 respiratory viral season and found a significantly higher risk of influenza with vaccination when influenza activity was high.