(355) Relative effectiveness (rVE) of the high-dose vs. standard-dose influenza vaccines in the French National health data system (SNDS) for people aged 65+ living in community, obtained using two approaches based on the propensity score

Background: The high-dose influenza vaccine (HD ) was recommended for the first time in France during the 2021/2022 season for individuals aged 65+.

Objectives: To evaluate the rVE of the HD vaccine compared to the standard-dose vaccine (SD) to prevent hospitalizations using two propensity score (PS) methods.

Methods: This retrospective cohort study was conducted using the SNDS. Individuals aged 65+ living in community who received a reimbursed influenza vaccine between September 1, 2021, and February 28, 2022 were included and followed from the vaccine dispensation date (index date) until: the end of the study period (June, 30 2022), admission to a nursing home, or death. The main analysis was based on a population matched on PS with a 1:4 ratio (HD vaccinated vs. SD vaccinated) and an exact constraint on age category, gender, region of residence, and vaccination week. An Inverse Probability of Treatment Weighting (IPTW) approach, creating a pseudo-population weighted with the inverse probability of being treated from the multivariable logistic model calculated for matching, with weights stabilized by the marginal probability of treatment,was used to study the robustness of the results. Hospitalizations related to influenza, pneumonia, and other respiratory conditions, as the primary diagnosis, were identified from the 14th day after index date until the end of the follow-up. Incidence rates (IRR) and rVE were estimated using a selected regression model (AIC minimization) among Poisson, negative binomial, classical, or zero-inflated models.

Results: In total, 7,832,853 individuals aged 65+ were vaccinated against influenza, with 431,643 receiving the HD and 7,401,210 the SD. 405,385 individuals who received the HD were matched with 1,621,540 individuals with the SD. An rVE in favor of the HD compared to the SD, equivalent to 23.3%, with an IRR of 0.77 (95% CI: 0.64-0.92) for influenza-related hospitalizations were found. IPTW was applied to a pseudo-population consisting of 463,665 HD vaccinated and 9,923,992 SD vaccinated individuals and results were quite similar: rVE was 22.2%, with an IRR of 0.78 (95% CI: 0.66-0.91) . Sensitivity analysis considering hospitalizations identified as the primary, related, and associated diagnosis and excluding COVID-related stays produced similar results.

Conclusions: The estimation of rVE is robust and stable regardless of the statistical method applied. These results demonstrate a better performance of HD vaccination in reducing hospitalizations due to influenza compared to SD in an assessment conducted on real-world data during the 2021-22 season.