A seroepidemiological study was carried out to explore the risk factors of a
measles outbreak that occurred among school children at a rural village (Li-Tse) in Taiwan. Among the 1166 participants, the percentage susceptible before the outbreak was 10.5% (122/1158) which was estimated as the sum of
measles IgG-negative (29/1158) and
IgM-positive (93/1166) individuals. Among 340 vaccinated children, 16 (4.7%) were
measles IgM-positive and 10 (2.9%) were
measles IgG-negative; therefore the
vaccine failure rate was estimated to be 7.6% (26/340) and
vaccine efficacy was 79.7% (95% confidence interval [CI] : 65.0-88.5). The most important risk factors for acquiring
measles infection were the presence of other
measles cases in the family (Odds Ratio [OR] = 32.5, P = 0.002) and the presence of more than two cases in a class (OR = 29.1, P = 0.003). The physician reporting rate was 6.1% (4/66), and the sensitivity of passive
measles surveillance was only 4.3% (4/93) by active serosurvey. A concomitant
rubella epidemic also amplified the inaccuracy of a passive reporting system based only on clinical diagnosis. Five children developed
measles IgM but did not experience any symptoms, indicating that asymptomatic
measles infection can occur. Our experience has highlighted three important areas for future
measles elimination: (1) the need for serological evaluation of vaccinees, particularly those who were born during the introduction of mass immunization; (2) improvement in
measles vaccine efficacy; and (3) further investigations on the role of asymptomatic transmission and susceptibles who remain after mass immunization.