Japan's national surveillance from January 2017 to October 2025 identified 288 IMD cases across pre-pandemic, pandemic and reopening phases. Cases fell from 48 in 2019 to one in 2021, then rose to 66 in 2024 and 68 by October 2025. Older adults consistently accounted for the most cases, adolescents clearly rebounded, and Tokyo rose more than Osaka/Kansai. No Expo-specific surge appeared. Cases and inbound travel had Spearman rho 1.000, although most infections were domestically acquired.
Key findings
- Cases changed 48 in 2019 to one in 2021, 66 in 2024 and 68 by October 2025; pandemic phase was lower and reopening higher; older adults led, adolescents rebounded; no Expo surge; travel correlation was rho 1.000.
Why this matters globally
The findings inform post-mobility surveillance and vaccination or risk communication by emphasizing age and geography rather than assuming a single mass event drives risk.
Thai researcher contribution
Y. Chen, Hirotake Mori and Rapeephan R. Maude are affiliated with Mahidol, Ramathibodi, MORU and Bumrungrad, linking Thailand-based clinical networks to Japanese infectious-disease epidemiology.
Limitations to consider
IMD is rare, so annual counts are small. A perfect correlation may reflect shared trends and few time points, not causal travel effects. Surveillance intensity may change, and serogroup, vaccination, contact and genomic linkage are not reported in the abstract.