Study protocol for a retrospective cohort study of real-world effectiveness of the VLA1553 (IXCHIQ®) chikungunya vaccine during an epidemic among healthcare workers in Reunion Island.

BACKGROUND

Real-world effectiveness data for the live-attenuated chikungunya vaccine VLA1553 (IXCHIQ®) remain limited because licensure relied primarily on immunogenicity and safety. The 2025 vaccination campaign at the University Hospital of Réunion offers an opportunity to evaluate effectiveness during an outbreak.

OBJECTIVES

HO-VLA-CHIK aims to estimate vaccine effectiveness against symptomatic chikungunya among hospital employees. Secondary objectives address laboratory-confirmed symptomatic infections, hospitalizations, chronic chikungunya, sick leave duration, adverse events following immunization, and determinants of vaccine uptake.

METHODS

This retrospective exposed-unexposed cohort study involves approximately 700 vaccinated and unvaccinated employees across two hospital sites. Structured electronic questionnaire data will be supplemented by occupational health registries, laboratory results, and medical records. A marginal Cox model with stabilized inverse probability of treatment weighting will estimate vaccine effectiveness, using calendar time and time-dependent vaccination status. Vaccinated person-time will begin 15 days after vaccination. Adjustment will use prespecified baseline confounders.

RESULTS

Ethical approval was obtained in May 2026. Recruitment and data collection are scheduled to end in December 2026. Analyses will follow data validation. Expected findings include estimates of vaccine effectiveness and descriptions of reported adverse events, persistent symptoms, and work absence. These findings may inform vaccination strategies and hospital workforce preparedness during future chikungunya outbreaks.