Assessment of the effectiveness of the universal varicella vaccination program in Toscana (Italy), in the period 2010-2013.

OBJECTIVE To assess the effectiveness of the varicella vaccination program in Toscana after one dose of vaccine, in the birth cohorts 2008-2011. DESIGN Varicella vaccine effectiveness (VE) was calculated using the "screening method", based on vaccine coverage (VC) at 24 months and proportion of vaccinated subjects among varicella notified cases (PVC), verified through the Local Health Units' (LHUs) immunization registries. Breakthrough varicella (BV) was defined as a case of varicella occurring in a child vaccinated ≥42 days before the date of disease onset. SETTING AND PARTICIPANTS The study was conducted in the 12 Tuscan LHUs and included all varicella cases notified in 2010-2013 in children of the birth cohorts 2008-2011. MAIN OUTCOME MEASURES BV cases; VE after one dose of varicella vaccine; time interval between varicella vaccination and symptom onset. RESULTS VC was 79.8%, VE reached 90.8%(95%CI 89.5%-92.0%) and the proportion of BV among notified cases was 26.6%. The median time interval between vaccination and symptom onset was 25 months. CONCLUSIONS The very low rate of BV cases among vaccinated children confirms the high effectiveness even of a single dose of varicella vaccine and does not support a change of the current immunization schedule.