Comparative varicella vaccine effectiveness during outbreaks in day-care centres

BACKGROUND: Routine varicella vaccination for children <11 months was introduced in Germany in 2004 with three different vaccine brands available. In 2008 and 2009, we investigated seven varicella outbreaks in day-care centres (DCC). METHODS: Varicella disease and vaccination status of 1084 children was reviewed to evaluate vaccination coverage (VC), brand-specific varicella vaccine effectiveness (VE), and risk factors of breakthrough varicella (BV, <42 days after vaccination). A case was defined as a child with acute onset of varicella attending one of the respective DCC at the time of outbreak. Children with a previous history of varicella, age>11 months, vaccinated at age>11 months or >42 days before disease onset or during the outbreak were excluded from VE and BV risk factors analyses (adjusted for gender, age and DCC). FINDINGS: Of 631 children with available vaccination information, 392 (62%) were vaccinated at least once. Overall VE among 352 children eligible was 71% (95% confidence interval (CI) 57-81, p>0.001) and differed significantly by disease severity and number of doses administered. Risk for BV was higher for 1 dose of Varilrix (RR=2.8, 95%CI 1.0-7.8, p=0.05) or Priorix-Tetra (RR=2.4, 95%CI 0.7-8.3, p=0.18) but lower for 2 doses of Priorix-Tetra (RR=0.5, 95%CI 0.1-2.7, p=0.41) than for 1 dose of Varivax. INTERPRETATION: Enhanced efforts to increase VC in Germany and 2 doses varicella vaccine might be successful to reduce the risk for BV. The evidence that VE and risk of BV are associated with vaccine brand needs further investigation.