The Use of School-Based Vaccination Clinics to Control Varicella Outbreaks in Two Schools

DISCUSSION

Varicella vaccination can avert or modify disease when administered up to 3 to 5 days after exposure^{6 ,7} and has been shown to be effective in controlling outbreaks in homeless shelters and the military.^{3 ,8} The Advisory Committee on Immunization Practices (ACIP) recently recommended vaccination for outbreak control and use of varicella vaccine postexposure.⁹ Our experience suggests that even with relatively high levels of coverage, school-based outbreaks occur and may be protracted. Our intervention was held 2 months and 1 month into the school A and B outbreaks, respectively. Although the earliest possible intervention would have prevented the most cases, our intervention was effective in increasing vaccine coverage, decreasing disease susceptibility, and shortening the outbreak. Additionally, vaccination may have protected unvaccinated susceptible children indirectly by interrupting transmission. In school A, the 11-day school break may have contributed to the resolution of the outbreak, although cases occurred throughout the period of school closure and cases continued to occur after the school break suggesting that transmission had not been interrupted.

The timing of the 7 cases reported after the vaccination clinic in school B suggests that 5 were incubating wild-type disease. Two other cases were both mild (<50 lesions). One child developed breakthrough disease after vaccination in 1997, and another child developed a modified case of varicella 12 days postvaccination, which could have been attributable to either wild or vaccine virus; we did not perform strain identification to determine the causative strain. No cases occurred among vaccinated children in school A after the clinic and the case that occurred in an unvaccinated child was presumed to have been exposed outside of school because onset occurred more than 1 incubation period after the last documented case in the school outbreak.

Although the sample size was small, this study suggests that 5% to 25% of parental reports of vaccination may be incorrect; the establishment of a policy to document varicella vaccine or varicella disease history in school records will provide more accurate information, which may be consulted rapidly during outbreak control efforts. Similarly, physicians should document in the medical records a child's positive history of disease in lieu of vaccination. This will become increasingly important as states implement day care and school requirements for varicella vaccination.

The high vaccine coverage seen in these school-aged children indicates an effective catch-up vaccination program in Philadelphia, especially in the youngest grade (kindergarten) in which coverage was 67% and 81% before the clinic. Among younger children 19 to 35 months of age, coverage in Philadelphia was 52% for July 1997 to June 1998—the highest in the country (Centers for Disease Control and Prevention, unpublished data, 1999).

School outbreaks are best prevented by implementation of ACIP recommendations for universal vaccination of children at 12 to 18 months of age and catch-up vaccination of older susceptible children. ACIP recently recommended that states institute requirements for child care and school entry to greatly increase coverage.⁹ In the absence of a school requirement, vaccination clinics in schools and day care centers can help reduce the number of susceptible children and shorten outbreaks when they occur.