The varicella vaccine was licensed in 1995, resulting in a decreased incidence of chickenpox among all vaccinated groups. However, given the continued occurrence of chickenpox outbreaks, there has been concern that the vaccine is less effective for preventing chickenpox than the 70 to 90 percent effectiveness predicted in prelicensure studies. In Utah, a stepwise requirement for all school-age children to be vaccinated against varicella was implemented in 2002. However, in the 2002–2003 school year, varicella outbreaks occurred in several schools, raising concerns about vaccine effectiveness. Haddad and colleagues assessed vaccine effectiveness in the affected community.
A questionnaire about vaccination history and varicella-like illness was administered to the parents of children attending the two affected elementary schools. Individual vaccination status was confirmed by consulting vaccination records and a statewide immunization information system. Varicella lesions in children with existing rash were analyzed by polymerase chain reaction testing. Attack rates in unvaccinated and vaccinated children were calculated.
Of 597 questionnaires distributed in one school and 952 in another, 558 (93 percent) and 924 (97 percent) were returned, respectively. According to the survey results, 91 percent of children were varicella immune, either by previous infection or by vaccination before the school outbreaks occurred. A total of 83 children with varicella illness were identified during the five-month outbreak. Parents reported 545 children as being vaccinated; of these, 432 (79 percent) were verified, and an additional 67 provided a precise vaccination date. Of the remaining children, 46 had only parental recall to substantiate the vaccination. Only 26 of the 38 children presumed to have breakthrough infection had confirmed evidence of vaccination; the remaining 12 children were reclassified as unvaccinated after further investigation. Ultimately, there were 48 primary varicella cases in unvaccinated children and 26 breakthrough cases in vaccinated children. The attack rates among unvaccinated children were 27 and 41 percent in the two involved schools, and 4 and 5 percent among vaccinated children. Vaccine effectiveness overall was 87 percent. Varicella illness was milder and of shorter duration in vaccinated children. Children who had received the vaccine within the previous five years, or at age greater than 18 months, were less likely to develop breakthrough varicella.
The authors conclude that the varicella vaccine is highly effective. Perception of decreased effectiveness was likely caused by parents’ mistaken impression that their children had been vaccinated and by the high number of cases among vaccinated children despite the low attack rate.