Real-world effectiveness of influenza vaccination amongst children aged under 5 in a tropical setting

Abstract Objectives Information on influenza vaccine-effectiveness amongst children in tropical regions with year-round influenza transmission remains scarce. We evaluated real-world influenza vaccine-effectiveness in a tropical setting amongst children aged under 5. Methods Population-based retrospective cohort study, including all Singaporean children aged under 5, from 1st May 2022-30th April 2024. Person-time was divided into vaccinated (<365 days prior) and unvaccinated categories (no prior vaccination/vaccinated≥365 days prior). Vaccination was classified using national registries. Overlap-weighted Cox regression was utilised to evaluate hazards-ratio (HR) of influenza hospitalisation between vaccinated/unvaccinated groups; vaccine-effectiveness was calculated as 1-HR. Waning of protection in vaccinated children was evaluated via logistic regression, using weeks 2–9 post-vaccination as reference. Results 196,593 children contributed 137,758 vaccinated and 245,952 unvaccinated observations over 80,839,605 person-days. Overall vaccine-effectiveness was 53% (95%CI=44%-60%). However, relative-vaccine-effectiveness (rVE) waned to 21% of baseline (95%CI=12-36%) by weeks 39-52 post-vaccination. In vaccine-naïve individuals, rVE declined to 11%(95%CI=3-36%) of baseline at weeks 39-52 following a single vaccine dose, whereas rVE declined to 13%(95%CI=4-43%) of baseline at weeks 39-52 after two-dose primary vaccination. Conclusion Vaccination conferred real-world protection amongst children aged under 5 in a tropical setting; though protection waned rapidly. Benefit of a two-dose primary-series might be more marginal in tropical settings.