Multicluster measles outbreak with a substantial proportion of modified cases in Tokyo, Japan, January-May 2026

Tokyo experienced a measles outbreak with 260 cases in early 2026 despite the country's elimination status. Adults aged 20-39 years were the most affected group. Modified measles occurred in 38% of cases and its incidence increased with prior vaccination. Although case numbers rose until April, the effective reproduction number R(t) fell below 1, indicating outbreak control. Multiple clusters of cases were identified, but many cases lacked epidemiological links, suggesting that modified measles is less frequently considered in differential diagnosis. Intensive contact tracing and epidemiological surveillance contributed to limiting disease transmission.