95% effective in preventing severe varicella and is most commonly about 80% effective at preventing all disease after expo-sure to wild-type VZV. This means that following close exposure to VZV, as may occur in a household or an outbreak setting in a school or daycare center, 1 of every 5 vaccinated children may develop breakthrough vari-cella. Exposure to VZV may also result in asymptomatic infection in the previously immunized child.In contrast, breakthrough disease is varicella that occurs in a per-son vaccinated >42 days before rash onset and is caused by wild-type VZV.The rash in breakthrough disease is frequently atypical, predomi-nantly maculopapular, vesicles are seen less commonly, and the illness is most commonly mild with">