Resurgence of measles in Japan, a highly vaccinated country: a risk assessment study.

BACKGROUND: Measles is a highly transmissible infectious disease that has re-emerged globally following disruption to vaccination programmes during the COVID-19 pandemic. This study quantitatively assessed the risk of measles outbreaks in Japan in 2026. METHODS: The effective reproduction number (Re) of measles was estimated using two different approaches. First, the cluster size distribution data were analysed using a branching process model, and possible associations between cluster size and characteristics of the primary case were explored. Second, the immune proportion of the Japanese population was estimated using vaccination coverage and seroepidemiological data. FINDINGS: Using cluster size data from 2025-2026, Re was 0.2 (95% confidence interval [CI] 0·1-0·3), with substantial overdispersion (dispersion parameter k 0·04, 95% CI 0·02-0·06). Larger clusters resulted from primary cases in unvaccinated people and children. In all age groups, more than 90% were considered immune, but adults aged 35-44 years were slightly more susceptible. Using the second method, Re was estimated at 1.2 (95% CI: 0·3-5·5) and potential cumulative incidence at 1,702,000 (0-6,994,000). INTERPRETATION: In early 2026, measles in Japan remains limited to minor outbreaks. If importation continues and people mix effectively, population immunity may be insufficient to prevent a major epidemic.