Outbreak of Varicella at a Day-Care Center despite Vaccination

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Volume 347:1909-1915 December 12, 2002 Number 24

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.,

Carraher, R.N., L. Baughman, Ph.D., M.P.H., Melinda Eaton,

D.V.M., Montero, M.D., and Jane Seward, M.B., B.S., M.P.H.

ABSTRACT

Background 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.

Methods 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.

Results 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 persons.

Source Information

From the Centers for Disease Control and Prevention, Atlanta (K.G.,

B.L., T.S., A.L.B., J.S.); the New Hampshire Department of Health and

Human Services, Concord (C.C., J.M.); and the College of Veterinary

Medicine, Washington State University, Pullman (M.E.).

Address reprint requests to Dr. Galil at 65 Hayden Ave., Lexington, MA

02421, or at karin.galil@....