SEROEPIDEMIOLOGY AND EVALUATION OF PASSIVE SURVEILLANCE DURING 1988- 1989 MEASLES OUTBREAK IN TAIWAN

A seroepiderniological study was carried out to explore the risk factors of a measles outhreak that occurred among school children at a rural village (Li-Tse) in Taiwan. Among the 1166 participants, the percentage susceptible before the outhreak was 10.5% (122/1158) which was estimated as the sum of measles lgG-negative (29/1158) and lgM-positive (93/1166) individuals. Among 340 vaccinated children, 16 (4.7%) were measles lgM-positive and 10(2.9%) were measles lgG-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 lgM 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 bom 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