[Field epidemiological study on a varicella outbreak among schoolchildren in Shenzhen].

OBJECTIVE

Both epidemiologic characteristics and transmission mode of a varicella outbreak among schoolchildren in Shenzhen city were studied and related control measures were discussed.

METHODS

Case definition was established. Case-finding and face-to-face investigation were conducted, followed by analysis on distributions of time, place and persons of the outbreak. Association between possible modes of transmission and the outbreak was explored. Retrospective cohort study was carried out.

RESULTS

The overall attack rate of this outbreak was 4.4% (122 of 2742). The attack rate among primary schoolchildren(8.3%, 118/1419)was higher than that in the middle-school children(0.3%, 4/1323). There were no statistically significant differences on the attack rates between male and female students. A total of 22 classes from the 9 grades were affected by this outbreak and the aggregation of varicella cases was found in classes. The highest attack rates was found in the students of fifth grade(23.7%), followed by from the third grade (13.4%). The main transmission mode appeared to be close personal contact. The outbreak, with four peaks of incidence, lasted 72 days. Data from the investigation suggested that the primary case was introduced into the school during military training involved by the students. Activities related to physical examination for all the schoolchildren seemed to have contributed to the spread of this disease among classes. Delayed isolation of cases appeared to be the major factor causing the spread of disease among classes. Intensive off-school activities or free community bus for children did not seem to be associated with the spreading of the disease. 52.5% (64/122) of the cases had received one-dose of varicella vaccine, with the median of onset after the vaccination as 7 years. The results demonstrated that one single-dose vaccine or long vaccination period were factors related to the insufficient immunity that causing the outbreak of disease.

CONCLUSION

Varicella outbreak has been one of the most grievous public health problems in schools, posting challenges on the implementation of isolation measures, in particular. Once the chain of transmission is in place, it is difficult to make the universal measures effective within a short period.