Epidemiological Investigation of Measles in Sera of Healthy People in Heilongjiang Province, China

SUMMARY: Serological samples of healthy people were collected to obtain the levels of measles an- tibodies in different groups of people in Heilongjiang Province, China. Using quantitative enzyme- linked immunosorbent assays to measure the antibody levels, we found the lowest antibody positive rate and the lowest geometric mean concentration values in healthy people aged 15-39. This group is the population at high risk for adult measles in Heilongjiang Province, and is the focus of measles elimina- tion work. The new challenges for the eventual elimination of measles will be to address immunization strategies in this segment of the population in order to control the incidence. Before the implementation of measles vaccinations, the incidence of measles was almost equivalent to the birth rate in Heilongjiang Province. Together with the promotion and application of measles vaccines, the sub- sequently successful immunization program resulted in a dramatic decline in the incidence of measles to 0.7 per million in 2004. Since 2005, the incidence of measles un- derwent a substantial rebound in that province. Further- more, the age at onset of emergence changed, as a con- siderable proportion of infected patients were adults, which suggested that the level of province-specific popu- lation immunity might be problematic. To investigate the immune status of the population of Heilongjiang Province, the sera of 126 healthy people in the Meilisi district were collected for epidemiological investigation by enzyme-linked immunosorbent assay (ELISA) and hemagglutination-inhibition (HI) tests. Expanded Program on Immunization (EPI) im- plementation was initiated in Heilongjiang Province in 1983. In the first years, the rate of measles vaccination was low, and the number of reported cases of measles was at a high level. After 1992, the coverage of measles vaccinations was stabilized at a high level, and there were a low number of reported cases of measles. The age of the target population used to calculate the coverage was 8-month-old children, an age after the first dose of basic measles immunization. The age com- position of measles incidence has changed, with a bi- directional shift transfer to an older age group and an infant group who have not yet reached the age of prima- ry immunization (Fig. 1).