The Measles Outbreak in Chikuhou District, Fukuoka, Japan, 1990: Correlation between Herd Immunity Level and Outbreak Size

A measles outbreak occurred in the Chikuhou district of Fukuoka, Japan from May to October 1990, during which 71 patients were cared for at the Itoda Public Hospital. Hospital records revealed a large outbreak in the adjacent town of Kanada. In order to characterize the outbreak, questionnaires were sent to all preschool‐age children in Itoda (73% effective response) and in Kanada (76% effective response) requesting information about their vaccination and/or history of measles. The number of patients was 22 (4%) in Itoda and 63 (14%) in Kanada, most of these being preschoolers, while the vaccination rate was 61% and 44%, respectively. The herd immunity levels in age‐specific groups were compared between the two towns. Before the epidemic, the immunity level of 1 year old children in Kanada, who showed the higher attack rate, was lower (18%) than that in Itoda (39%), while after the epidemic it rose above 60% in both towns. When we studied the correlation between the attack rate and the vaccination rate, or the number of children susceptible to measles (susceptibility rate) in each preschool, the attack rate correlated negatively with the vaccination rate (correlation coefficient [CC] = ‐ 0.818; P < 0.01), and positively with the susceptibility rate (CC 0.860; P < 0.01). The regressed equation on the correlation indicated that the immunity level should be more than 70% to keep the attack rate under 5% in preschoois. After the epidemic, the immunity levels of all preschoolers reached above 70%. Early vaccination should be given to at least 70% of young preschoolers in order to prevent a large outbreak of measles under the present vaccination program in Japan.