Effectiveness and failure rate of the varicella vaccine in an outbreak in Jiangsu, China: a 1:2 matched case-control study.

Background

The varicella vaccine is not included in the national childhood immunization schedules in China, although one-dose varicella vaccine has been suggested for susceptible children aged 1-12 years in Jiangsu Province. However, varicella epidemics and outbreaks are frequently reported. We investigated a varicella outbreak in an elementary school to explore the risk factors for varicella transmission and vaccine failure.

Methods

A 1:2 matched case-control study was carried out. Participant data were collected with standardized questionnaires. For each case, we enrolled two controls: a subject with high exposure in the same classroom as the case and a subject with low exposure in a different classroom. Data regarding vaccination status and medical and exposure histories were analyzed.

Results

Fifty-one cases were reported during the outbreak; 26 cases (51%) were breakthrough varicella. Varicella vaccine immunization history (P < .001, OR = 0.19, 95% CI = 0.08-0.45) and the presence of siblings (P = .037, OR = 0.45, 95% CI = 0.21-0.95) were protective factors in preventing varicella infection. Contact with varicella patients increased the risk of varicella infection (P = .028, OR = 3.39, 95% CI = 1.14-10.09). Breakthrough varicella cases tended to present a milder rash (P = .049), fewer complications (P = .02), fewer rash sites (P = .02) and a shorter duration of active lesions (P = .001). One pneumonia case and one encephalitis case were reported in breakthrough cases. Age <15 months at the time of vaccination increased the risk of breakthrough varicella (P = .012). The adjusted vaccine effectiveness was 81%.

Conclusions

One-dose varicella vaccine is effective at alleviating clinical manifestations. The moderate coverage provided by one dose cannot prevent varicella outbreaks, and vaccination after 15 months of age should be considered in the immunization schedule; a two-dose strategy is highly recommended.