Fatal Adverse Event After VLA1553 Chikungunya Vaccination in an Elderly Patient: A Case Report From Reunion Island.

We report a fatal case of febrile encephalopathy in an 84-year-old man following administration of the live-attenuated chikungunya vaccine IXCHIQ® (VLA1553, Valneva SE) during the 2025 outbreak in Réunion Island. The patient, previously autonomous with stable comorbidities, developed fever, asthenia, and polyarthralgia 3 days post-vaccination. His condition rapidly deteriorated, with confusion, acute kidney injury, and hemodynamic instability requiring intensive care. Chikungunya virus RNA was detected in both serum and cerebrospinal fluid, with sequencing confirming the presence of the vaccine strain in both compartments. Despite broad-spectrum antimicrobials, antiviral therapy targeting herpesviruses, and hemodialysis, his neurological status worsened and he died 14 days after symptom onset. This case is the first fatal adverse event officially recognized by French health authorities as plausibly related to VLA1553 vaccine. It raises concerns regarding potential neuroinvasive disease following vaccination in elderly individuals and highlights the importance of close clinical and virological evaluation in post-vaccination adverse events during outbreak settings.