Post-pandemic outbreak of measles seen in Japan, 2024

An imported case of measles was reported on 1 March 2024 in Japan, followed by additional cases involving individuals who took the same flight as the index case. In addition to post-pandemic real-time assessment of the transmission dynamics, we aimed to clarify the epidemiological characteristics of the 2024 measles outbreak in Japan.

We analyzed confirmed-case data based on notifications from public health centers. To estimate the effective reproduction number (R_t), we retrieved a serial interval distribution from an outbreak dataset in the United Kingdom. Estimating the infectiousness profile as a function of time since illness onset, we back-projected the time of secondary transmission to compute the R_t. We reconstructed the transmission tree based on contact tracing information.

As of 13 May 2024, a total of 24 measles cases were notified during 2024, among which 13 were considered to have acquired the infection from the index case. The estimated R_t peaked at 14.1 (95 % CI: 1.2-33.4) on 23 February, the day before the index case self-isolated. R_t subsequently declined, and public announcements of diagnosed cases were made by the affected prefectures, raising awareness about the measles outbreak.

An imported measles case led to a super-spreading event, but further secondary transmission was prevented; the index case self-isolated and awareness was raised about the outbreak by public health centers. The present simple method of computing the R_t is useful for monitoring future outbreaks of any directly transmitted infectious disease.