Outbreak of varicella in preschool children despite one-dose vaccination

Özet

In Turkey, a single-dose varicella vaccine was introduced into the NationalImmunization Program in 2013. Before this implementation, varicella vaccinehad been available in the private sector since 2000. However, varicella outbreakscontinued to occur in preschools and elementary schools. We investigated avaricella outbreak to estimate the effectiveness of 1-dose varicella vaccine andto evaluate potential risk factors for breakthrough disease. This study wascarried out during a varicella outbreak in 3 preschools in İzmir, Turkey, in April2016. Using questionnaires, data including children’s medical and vaccinationhistories were collected from their parents. Attack rates in vaccinated andunvaccinated children were calculated and the analyses of vaccine effectivenessand of risk factors for breakthrough disease were conducted.A total of 124 children were enrolled in the study. Of the 124 children, 77(62%) had received 1-dose varicella vaccine before the outbreak. Varicelladeveloped in 34 of 124 children during the outbreak, and 18 of them (53%)had breakthrough varicella. The attack rate was 23.4% among vaccinatedchildren and 34% among unvaccinated children. The effectiveness of singledose varicella vaccine was 33.6% against varicella disease of any severity and82.5% against moderate or severe varicella. Children vaccinated 5 or moreyears before the outbreak had 3.5 times the risk of disease than those whohad been vaccinated more recently (OR 3.5 [95% CI, 1.08-11.5]); p= 0.046).Age at vaccination (<15 months vs.≥15 months) and the brands of varicellavaccine were not associated with the increased risk of breakthrough varicella.Our study suggests that one-dose of varicella vaccine is not sufficient toprevent school outbreaks. A 2-dose varicella vaccination program may helpto prevent varicella outbreaks and achieve effective control of the disease.