February 2000
DURHAM, N.C. - Varicella vaccine (Varivax, Merck) is effective in preventing or modifying wild-type varicella among children in a day care setting, according to a study reported in Pediatric Infectious Disease Journal.
Researchers documented the effectiveness of varicella vaccine during a 19-month study of children attending 11 day care centers or schools in North Carolina. Data showed 134 cases of varicella occurred in unvaccinated children and 11 cases occurred in vaccinated children.
The centers served 1,100 to 1,500 children ages 5 and younger. Questionnaires were sent to parents of children enrolled in any of the 11 centers, and were distributed four times throughout the study. Eligible cases were determined by a typical varicella rash or a modified varicella-like illness as reported by a teacher or parent.
According to the study, the number of skin lesions in vaccinated children was less than the number in unvaccinated children, as were the number of day care days missed. No significant difference was found in the frequency of fever, cough or runny nose.
Throughout the study, vaccine effectiveness against all forms of varicella disease was 83%. Researchers concluded that if children in the North Carolina study represent all children in the day care setting, then vaccine effectiveness against moderate-to-severe disease is 100%.
"We're monitoring the vaccine in real life, with real kids in a real day care center. It confirms once again how good the vaccine is," said Dennis Clements, MD, Ph.D., medical director of primary care at Duke University, Durham, N.C.
A related, post-marketing evaluation of safety and effectiveness studied a population of 89,753 children and adults. Potential adverse effects were identified by the number of hospitalizations, emergency room and clinic visits. An analysis of the events, adjusted for the concomitant administration of vaccines like measles-mumps-rubella (MMR, M-M-R II, Merck), revealed none of the adverse associations were statistically associated with receipt of the varicella vaccine.
According to the study, only mild gastroenteritis and negative evaluations for sepsis were thought to be possibly associated with the receipt of varicella vaccine. Rates of varicella-like rash and breakthrough cases were low and consistent with the rates observed in prelicensure evaluations.
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Because of its relatively new status, varicella vaccine was often given alone during the study as part of a "catch up" program. In the 21-month study, 60.8% of the vaccine doses were given alone, and 22.6% were given with MMR, 12.9% with oral poliovirus or inactivated poliovirus vaccine, 10.7% with diphtheria-tetanus-pertussis vaccine, 77% with diphtheria-tetanus-acellular pertussis vaccine, 7% with hepatitis B vaccine and 3.7% with a Haemophilus influenzae type b vaccine.
Only two deaths were reported within 60 days of vaccination, but neither was associated with vaccine administration.
Varicella infects more than 90% of children before the age of 10. The Oka strain of live-attenuated varicella virus was licensed for use in healthy children in 1995. "This vaccine has been extensively used in millions of children and we were able to report on almost 100,000 doses regarding its safety," said Steven B. Black, MD, codirector of the Kaiser Permanente Vaccine Study Center and a member of the editorial board of Infectious Diseases in Children.
For more information:
- Clements D, Moreira S, Coplan P, et. al. Post-licensure study of varicella vaccine effectiveness in a day-care setting. Pediatr Infect Dis J. 1999;18:1047-1050.
- Black S, Shinefield H, Ray P, et. al. Postmarketing evaluation of the safety and effectiveness of varicella vaccine. Pediatr Infect Dis J 1999;18:1041-1046.
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