Objective.

1) To describe the postexposure effectiveness of varicella vaccine in a homeless shelter; and 2) to demonstrate an effective public health intervention and its implications.

Design.

A prospective observational study.

Setting.

A women and children's shelter in Philadelphia with 2 cases of varicella before intervention.

Outcome Measures.

Varicella in vaccinated and unvaccinated shelter residents; vaccine effectiveness for prevention of varicella when administered after exposure among children <13 years of age.

Results.

Sixty-seven shelter residents received varicella vaccine after exposure, including 42 children <13 years of age. One child who was unvaccinated developed varicella, but no vaccinated child developed typical disease. Vaccine effectiveness was 95.2% (95% CI, 81.6%–98.8%) for prevention of any disease and 100% for prevention of moderate or severe disease among the children <13 years of age.

Conclusion.

When used within 36 hours after exposure to varicella in a setting where close contact occurred, varicella vaccine was highly effective in preventing further disease. This study provides support for the recent recommendation by the Advisory Committee on Immunization Practices to administer varicella vaccine after exposure: this practice should minimize the number of moderate or severe cases of disease and prevent prolonged outbreaks.

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