Seroepidemiology of varicella among elementary school children in northern Taiwan.

BACKGROUND/PURPOSE: In Taiwan, varicella vaccine was included in the expanded program of immunization since 2004. A seroepidemiologic study in the postvaccine era is helpful to evaluate the efficiency of current varicella vaccination strategies.

METHODS

We used a multistage stratified systematic sampling design to classify 29 administrative districts of New Taipei City into five strata. In 2013, a total of 936 students from 14 primary schools were recruited and had blood drawn for serology tests for varicella-zoster virus-immunoglobulin-G via indirect chemiluminescence immunoassays. A history of clinical varicella and information on varicella vaccination status were obtained.

RESULTS

Overall, the seroprevalence was 64.1%. For the five strata, the seropositive rate ranged from 54.2% (Stratum 5) to 71.7% (Stratum 2) with no significant difference. For each participating school, the seropositive rate ranged from 44.4% to 72.9% with a statistically significant difference (p < 0.005). For school children in each grade, seropositive rate increased significantly from 53.2% for Grade 1 to 71.8% for Grade 3 (p = 0.005) and increased steadily from 61.2% for Grade 4 to 71.2% for Grade 6 (p = 0.17). A positive correlation was observed between the seropositive rate and geometric mean titers (p = 0.035). Geometric mean titers and the rate of a history of clinical varicella were positively correlated with increasing class grades.

CONCLUSION

Nine years after the introduction of the varicella vaccine into the expanded program of immunization in Taiwan, around two-thirds of elementary schoolchildren were seropositive for varicella-zoster virus. Further surveillance studies on clinical varicella cases are worthwhile to determine whether a second dose of varicella vaccine is needed in Taiwan.