Real-world effectiveness of influenza vaccination and subsequent waning in a tropical setting: a retrospective cohort...

Information on influenza vaccine effectiveness in tropical regions with year-round influenza transmission remains scarce. We evaluated real-world influenza vaccine effectiveness in a tropical setting following the resurgence of influenza activity post-COVID-19, assessing the impact of repeated vaccination on vaccine effectiveness and waning.

Population-based retrospective cohort study, including all adult Singaporeans (1 May 2022-30 April 2024). Person-time was divided into vaccinated (<365 days prior), and unvaccinated/vaccinated ≥365 days prior, using the National Immunisation Registry. Cox regression, with overlap weights applied, was utilized to evaluate the hazard ratio (HR) of influenza hospitalisation between vaccinated/unvaccinated groups. Vaccine effectiveness was calculated as 1 HR. Relative waning of vaccine effectiveness in older adults (≥65 years) was separately evaluated via logistic regression, using weeks 2 to 9 post-vaccination as reference, and stratified by prior doses (1/2/≥3 doses) in the past 2 years.

In total, 3 445 961 adult Singaporeans (≥18 years) were included, comprising 6 245 014 vaccinated/unvaccinated observations >1 376 225 223 person-days. Overall vaccine effectiveness against influenza hospitalisation was 37% (95% CI, 32-42%) amongst older adults (≥65 years). However, relative vaccine effectiveness waned to 43% of baseline (95% CI, 34-54%) by 18 to 25 weeks post-vaccination. Receipt of two or more vaccine doses in the preceding two years was associated with lower odds of vaccine failure (adjusted odds ratio = 0.81 (95% CI, 0.71-0.91%)); no significant waning was observed in those who cumulatively received three or more vaccine doses in the past two years.

Vaccine effectiveness against influenza hospitalisation was estimated at 37% in a tropical setting with year-round influenza transmission, although protection waned by ≥ 50% 6 months postvaccination. No significant waning was observed in those who received three or more vaccine doses in the past two years; increased revaccination frequency may mitigate waning in tropical settings.