Seroepidemiologic Study of Measles Outbreak in a Primary School, Youngduk

Lee, Jin Soo; Kim, Jee Hee; Na, Byung Kuk; Yoon, Sin; Kim, Jin Ah; Lee, Joo Yeon; Kang, Chun; Park, Sue Kyung; Cheong, Hae Kwan; Kim, Woo Joo

Abstract

BACKGROUND: Nation-wide measles outbreaks occurred from 2000 to 2001 in Korea. Since, mass vaccination campaign for measles eradication among children and adolescents was implemented in May-August of 2001. Even in a highly vaccinated community, measles cases occur among vaccinees and there might be different serological responses among unvaccinees. In addition, the primary and secondary vaccine failures are not rare in the mass-vaccination era. Therefore, this study was undertaken to evaluate the effectiveness of serologic method for diagnosis of measles among the vaccinees and to estimate the vaccine failure rate among students with relatively high vaccination rate during the measles outbreak in a primary school.
METHODS
In March 2000, the measles outbreak occurred in a primary school in Youngduk county, Kyung-sang-buk Do. Questionnaires about clinical manifestations of measles and history of measles vaccination were given out to in 209 3rd and 5th grade students among whom many measles cases occurred. The paired sera, which were taken from all students during March and May 2000, were tested for IgM and IgG measles antibody by using Enzygnost Anti-Measles IgG and IgM (Dade Behring, Germany).
RESULTS
The questionnaires and serum samples were obtained from 145(69.4%) out of 209 cases. The immunization rate of measles vaccine was 80.7% (117/145). During the measles outbreak, 25 (17.2%) out of 145 cases with previous history of measles vaccination were diagnosed as measles, based on clinical definition. Among 17 cases (17/25, 68.0%) with paired samples out of 25 measles cases, 4 cases (4/17, 23.5%) were diagnosed by seroconversion of IgG, but negative IgM antibody, and they all had received measles vaccination previously. Among 117 cases with previous history of measles vaccination, 47 (40.2%) cases without measles symptoms showed four-fold increment of IgG titer. The primary and secondary failure rates of measles vaccine were 6.0% (7/117) and 1.7% (2/117), respectively.
CONCLUSION
Among symptomatic measles cases with previous history of measles vaccination, 23.5% were diagnosed only by seroconversion of IgG measles antibody. In a highly vaccinated country, the accuracy of measles serodiagnosis can be increased with the IgG test of convalescent serum sample in addition to IgM antibody test. Among vaccinees, secondary immune response was 40.2%, suggesting that non-specific measles symptoms might occur among them. Therefore, further studies on the clinical definition of measles cases, vaccine failure rate and protective antibody titers should be continued in order to effectively conduct measles eradication campaign.