Publication date: May 12, 2025

Chikungunya virus (CHIKV) is a mosquito-transmitted, arthritogenic alphavirus that causes sporadic outbreaks of often debilitating rheumatic disease. The recently approved CHIKV vaccine, IXCHIQ, is based on a live-attenuated CHIKV strain (VLA1553), with viraemic vaccine recipients theoretically able to transmit VLA1553 to mosquitoes with ensuing onward transmission. We thus evaluated VLA1553 transmission from artificial blood meals to Aedes albopictus mosquitoes, and onward transmission to mice. Female A. albopictus mosquitoes were fed on defibrinated sheep blood containing wild-type CHIKV (viral titre: 7. 50 logCCID/mL) or VLA1553 (viral titres: 7. 85, 5. 72, 4. 58, and 3. 79 logCCID/mL). Viral titres in mosquito bodies and saliva were determined using CCID assays 7-8 days after the blood meal. After providing CHIKV or VLA1553 (viral titres ~ 7-8 logCCID/mL) in blood meals to mosquitoes, infected mosquitoes were fed on highly susceptible Irf3/7 mice (n = 3 per group). Data were re-analysed using the same reverse transcription quantitative polymerase chain reaction (RT-qPCR) as for an earlier VLA1553 phase 1 clinical trial, to allow correlations between blood meal titres and viraemia in vaccine recipients. Mosquito body viral titres were significantly higher (P 

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