Reports of outbreaks of varicella in populations that were at least partially vaccinated and that reported vaccine effectiveness and measures of its variance (or data that would enable calculation of this), were eligible for inclusion in the review. Outbreaks of varicella in restricted subgroups were excluded.
The median age of the included participants, where reported, ranged from four to eight years and the percentage population vaccinated ranged from 30 to 96%. The authors reported that all outbreaks included in the analysis were in children in day-care centres or elementary school. In most studies, the cases were defined as having developed an acute maculo-papulo-vesicular rash in the outbreak period, with only a few being microbiologically confirmed. Outcomes that were confirmed by laboratory diagnosis, parental or physician diagnosis within the outbreak setting were also eligible for inclusion
Studies were selected independently by two reviewers.