Outbreak of Varicella at a Day-Care Center despite Vaccination
Karin Galil, M.D., M.P.H., Brent Lee, M.D., M.P.H., Tara Strine, M.P.H.,
Claire Carraher, R.N., Andrew L. Baughman, Ph.D., M.P.H., Melinda Eaton,
D.V.M., Jose Montero, M.D., and Jane Seward, M.B., B.S., M.P.H.


 In seven studies of the effectiveness of the varicella vaccine
conducted since it was licensed, the effectiveness was 71 to 100 percent
against disease of any severity and 95 to 100 percent against moderate
and severe disease. We investigated an outbreak of varicella in a population
of children with a high proportion of vaccinees who were attending a
day-care center in a small community in New Hampshire.

Using standardized questionnaires, we collected information about
the children's medical and vaccination history from parents and health
care providers. The analysis of the effectiveness of the vaccine and of risk
factors for vaccine failure was restricted to children who were enrolled
in the day-care center continuously during the outbreak and attended for one
week or more and who were cared for in the building that represented the
epicenter of the outbreak, since transmission was not documented in a
second building.

Varicella developed in 25 of 88 children (28.4 percent) between
December 1, 2000, and January 11, 2001. The index case occurred in a
healthy child who had been vaccinated three years previously and who infected
more than 50 percent of his classmates who had no history of varicella. The
effectiveness of the vaccine was 44.0 percent (95 percent confidence
interval, 6.9 to 66.3 percent) against disease of any severity and 86.0
percent (95 percent confidence interval, 38.7 to 96.8 percent) against
moderate or severe disease. Children who had been vaccinated three years
or more before the outbreak were at greater risk for vaccine failure than
those who had been vaccinated more recently (relative risk, 2.6 [95 percent
confidence interval, 1.3 to 5.3]).

Conclusions In this outbreak, vaccination provided poor protection
against varicella, although there was good protection against moderate or severe
disease. A longer interval since vaccination was associated with an
increased risk of vaccine failure. Breakthrough infections in vaccinated,
healthy persons can be as infectious as varicella in unvaccinated