Varicella vaccination was found to be highly effective during an outbreak of varicella among elementary school children in Utah, reported Maryam B. Haddad of the Centers for Disease Control and Prevention, Atlanta, and associates.
The outbreak occurred from October 2002 until February 2003 in two schools, one with 597 students (school A) and another with 952 students (school B). Parents returned a questionnaire about their children's health. Those who reported varicella symptoms in their children were interviewed, their children's vaccination records verified, and they were asked to submit any existing lesions for polymerase chain reaction (PCR) testing.
Utah implemented a new varicella vaccination requirement for kindergarten children in 2002.
During the outbreak, 57 unvaccinated and 26 vaccinated children were reported to have varicella, with 17 more cases among household contacts, they said. PCR analysis found wild-type varicella in five unvaccinated and three vaccinated children (two other vaccinated children had specimens insufficient for testing). Nine unvaccinated children with varicella reported it as a second occurrence of the disease.
In school A, 27% of the 66 unvaccinated children acquired varicella, while only 4% of the 223 vaccinated children did. In school B, 41% of the 74 unvaccinated children reported varicella, while only 5% of the 348 vaccinated children did (Pediatrics 2005;115:1488–93).
The varicella vaccine overall was 87% effective. It was 90% effective against moderate or severe disease in school A and 99% effective in school B. Among the nine unvaccinated children with a history of varicella, the attack rate was 0.4% in school A and 1.4% in school B. Mild varicella was more common among vaccinated children (69%) than unvaccinated children (15%).
Risk factors for breakthrough varicella included a history of eczema (3.8 times greater risk), time since vaccination (relative risk 3.0 if vaccinated 5 or more years before the outbreak), and age at vaccination (relative risk 2.6 if vaccinated at age 18 months or less).
Among the 163 children vaccinated 5 or more years before the outbreak, children vaccinated at age 18 months or less were 9.3 times more likely than those vaccinated after age 18 months to develop breakthrough varicella.