Chikungunya fever among Japanese travelers: monthly and yearly trends in imported cases, 2011–2023

Objectives: Chikungunya poses an increasing threat to people living in endemic areas and to travelers from non-endemic regions. This study descriptively characterized imported chikungunya cases in Japan and explored the relationship between local incidence in destination areas and infection risk among Japanese travelers. Methods: First, the descriptive features of imported chikungunya cases in Japan from 2011 to 2023 were examined. Second, monthly and yearly trends in the incidence of imported chikungunya cases among Japanese travelers were compared by destination with incidence in the corresponding destination country. Third, destination-specific incidence among Japanese travelers was compared between high- and low-transmission seasons in each destination country. Results: Of 155 imported chikungunya cases in Japan during 2011-2023, 112 (72.3%) were associated with travel to Myanmar, India, Indonesia, the Philippines, or Thailand. The yearly chikungunya importation rate among Japanese travelers returning from Indonesia and Thailand was correlated with the corresponding rate among local residents. The monthly incidence of imported cases during the high-transmission season was 12.3, 7.4, 12.0, and 4.0 times higher than that during the low-transmission season among travelers returning from Myanmar, India, Indonesia, and the Philippines, respectively. Conclusion: The risk of imported chikungunya infection among Japanese travelers reflected the local epidemiology of chikungunya in destination countries, including both seasonal and long-term patterns. Imported-case data may support risk assessment in destination countries and contribute to traveler-specific prevention, improved diagnosis, and a reduced risk of autochthonous outbreaks.