A susceptible cluster in a religious camp allowed 2018-2019 measles outbreak in Japan: resurgence of a formerly eliminated virus.

Authors

Tetsuro Kobayashi, Hiroshi Nishiura

Published in

Frontiers in public health. Volume 14. Pages 1813637. Epub Jun 08, 2026.

Abstract

We analyzed the 2018-2019 outbreak of measles in Japan, which started with a gathering at a religious youth camp in Mie, a prefecture located in the central part of Japan.
A retrospective secondary analysis of cases from December 2018 to January 2019 was carried out. The index case was a Japanese man in his 20s living in Wakayama, a prefecture adjacent to Mie Prefecture. As part of his religious activities, he attended a youth camp in Mie Prefecture held from December 23 to 30, 2018. There were 28 identified secondary cases within the camp deriving from the index case, 2 additional household secondary transmissions from the index after the camp, and 31 additional reported cases from the attendees of the camp, for a total of 62 cases. Analyzing the publicly available data, including the first and last dates of exposure for each case, we calculated the incubation period, relative frequency of secondary transmission, estimated time of exposure during the camp, and effective reproduction number.
The incubation period had a mean of 11.2 days and variance of 5.3 days². When the relative frequency of secondary transmission was assumed to follow a gamma distribution, the mean and variance were estimated to be 3.0 days and 0.3 days², respectively. In the religious camp, the interquartile range (25th to 75th percentile) of infection time was 1.8 to 2.4 days prior to the onset of the primary case. The effective reproduction number during the religious camp was in the range of 30 to 32, which quickly dropped to <1 after the camp.
We successfully show that a large outbreak of measles may happen in a cluster of unvaccinated individuals, especially in a closed environment such as a teenage camp. Strengthening routine immunization program by targeting unvaccinated populations would be vital.

PMID:
42338521
Bibliographic data and abstract were imported from PubMed on 24 Jun 2026.

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