Impact of respiratory syncytial virus immunization in older adults: the importance of a valid burden estimation

IntroductionIn the province of Quebec, Canada, a respiratory syncytial virus (RSV) immunization program restricted to seniors residing in long-term care facilities (LTCF) and those aged [≥]75 years from private senior residences (PSR) was implemented in Fall 2024, with approximately 65% vaccine uptake in the targeted populations. Despite high vaccine effectiveness, the real-world impact of the RSV immunization program on population-level averted hospitalizations and the influence that RSV diagnostic test performance might have remains unknown. This study aimed to evaluate the impact of the first season (2024-25) of the intervention among older adults in Quebec while accounting for laboratory test performance. MethodsA prospective hospital-based surveillance (HospiVir) and health administrative databases were used to assess the intervention impact. Observed to expected approach and before/after ecological design using a difference-in-difference Poisson regression, both accounting for seasonal variations and trends in comparison groups, were used. Hospitalization incidence rate estimates based on hospital-based surveillance were adjusted for the laboratory test performances using Lash equations. ResultsBased on HospiVir, a 49% (95% confidence interval (CI): -80- -5) decrease in RSV-associated hospitalizations was observed among adults aged [≥]75 years residing in PSR. This decrease was 23% (95% CI: -38-66) in the overall population [≥]75 years. A smaller decrease for the overall population [≥]75 years was observed using health administrative data, but it did not reach statistical significance (10%; 95% CI: -7-23). The impact of the immunization program did not change following adjustment for diagnostic test performance, while the absolute burden of RSV-associated hospitalizations varied according to the proportion of RSV test positivity. ConclusionsThe 2024-25 RSV immunization program was associated with an almost 50% reduction in hospitalizations in residents of PSR aged [≥]75 years. The magnitude of this reduction was roughly twofold lower when considering the overall population aged [≥]75 years. Expanding the target population and strengthening vaccine promotion efforts to improve uptake are both essential to enhance the population-level impact of the RSV immunization program.