Article ID: JJID.2018.083
Modified measles (M-Me), characterized by milder symptoms than typical measles (T-Me), has been increasing in Japan, but M-Me dominated outbreak has not been investigated sufficiently worldwide. During March–April 2017, there was the largest importation-related outbreak of measles with genotype D8 in Yamagata, Japan, after Japan had achieved measles elimination in 2015. We confirmed 60 cases by detecting measles virus (MeV) genome, among whom 38 M-Me and 22 T-Me cases were identified. Thirty-nine cases (65.0%) were between 20–39 years of age. Three out of seven primary cases produced 50 transmissions, in which each patient caused 9–25 transmissions. They were aged 22–31 years with no vaccination, developed T-Me, and kept contact with the public during their symptomatic periods. Considering that M-Me is generally caused by vaccine failure, some individuals with insufficient immunity for MeV might exist in Japan. Accordingly, additional doses of measles vaccine may be needed especially for people aged 20–39 years to prevent measles importation and endemicity. Furthermore, to correctly and rapidly diagnose measles, especially to identify patients who could become primary cases, efforts to detect all measles cases using epidemiological and genetic approaches should be made in countries where measles elimination had been achieved.