Field epidemiology investigation on measles outbreak started from a junior high school at Kita Ibaraki City

Abstract We studied measles outbreak in Ibaraki Prefecture in spring 2002 as members of Field Epidemiology Training Program Japan (FETPJ). Of 84 cases diagnosed by not laboratory test but clinically, 67 (79.8%) were junior high school students, 9 (10.7%) were other students, and 8 (9.5%) were ordinary adult and infant citizens. Of the 84, 46 (54.8%) had been vaccinated. Most did not show typical Koplik spots. The city in which the school was located promoted vaccinations for infants and children aged 7.5 years old to grade 1 in High school. Questionnaire given to junior high students were detected 86 cases, of whom 57 (66.3%) were male. Overall, 4 peaks of clusters were observed in an epidemic curve, among which graduates' farewell parties and graduation ceremonies were the most common opportunities for measles virus exposure. The overall vaccination rate at school was 82.2%, vaccine efficacy extremely low at 72.5%, and vaccine failure high at 15.2%. Symptoms among those vaccinated were significantly milder than those not vaccinated. Immunity of those vaccinated as infants may have been decreased due to scarcity of measles cases in the last 10 years. In such situations--much less in typical measles among susceptible non affected and non vaccinated subjects--atypical or mild measles may be difficult to diagnose. These findings may keep clarify the need to introduce two-dose measles immunization in Japan.