Metaanalysis of vaccine effectiveness in varicella outbreaks.

There is a number of reports on varicella outbreaks in populations where a one dose varicella immunization program has been implemented. We performed a metaanalysis to provide a summary vaccine effectiveness (VE) estimate and to assess the possible impact of waning immunity by means of a subgroup analysis by time since immunization. We found a VE of 72.5% (95% CI 68.5-76.0) derived from 3157 children in 14 publications. Immunization coverage of the respective population was unrelated to VE. All studies (n=4) that allowed the computation of VE over time since immunization showed a substantial decrease. In total, waning immunity was assessed by nine studies. Two reported no relation between VE and time since immunization without specifying how this had been assessed. Seven studies calculated relative risks for contracting varicella after prolonged as compared to a shorter period since immunization and reported an increased relative risk for prolonged periods. In conclusion, this metaanalysis confirms a limited effectiveness of one dose of varicella vaccine and points to waning immunity as an important causal factor. Waning might also be an issue with the newly recommended two dose vaccination schedule. Sustained surveillance for varicella outbreaks in populations with varicella immunization programs therefore is mandatory.