Post-marketing safety evaluation of the live-attenuated chikungunya vaccine (IXCHIQ)

Abstract

Background: IXCHIQ (VLA1553) is a licensed live-attenuated chikungunya vaccine. A post-marketing safety review, including the EU Article 20 referral to the European Medicines Agency (EMA) and its Pharmacovigilance Risk Assessment Committee (PRAC), was initiated following early real-world use during the 2024-2025 La Réunion outbreak. This analysis integrates global serious adverse event (SAE) data with major regulatory assessments to inform clinicians and policymakers.

Methods: Serious adverse events reported from 9 November 2023 to 31 August 2025 were evaluated across major pharmacovigilance systems (EudraVigilance, Vaccine Adverse Event Reporting System, Canada Vigilance, MHRA) and cases received by the marketing authorisation holder (MAH).

Results: By 31 August 2025, approximately 55,900 doses had been administered worldwide, yielding 35 SAEs (6.3 per 10,000 doses). One-third of doses (33%) were administered in adults ≥65 years, who accounted for 77% of SAEs, reflecting the older, multimorbid population initially targeted during outbreak response in La Réunion. The median age of persons with SAEs was 73 years; 91% had comorbidities. Most events occurred within 7 days and included systemic chikungunya-like reactions (15), neurological (13), cardiac (3), and renal (3) manifestations. Three fatalities occurred in older men with multimorbidity; one case of vaccine-strain encephalitis was considered probably related to vaccination. No manufacturing or batch-related issues were identified.

Conclusions: Early post-marketing SAEs occurred mainly in older adults with multimorbidity, reflecting reactogenicity patterns expected with live-attenuated vaccines, while serious events were uncommon in adults <65 years relative to doses administered. Regulatory reviews concluded that IXCHIQ's overall benefit-risk balance remains favourable within authorised and nationally recommended populations, and product information was updated to reinforce warnings, precautions, and contraindications.

Keywords: ICSR; Passive surveillance; Reactogenicity; Signal detection, pharmacovigilance.