Varicella vaccine effectiveness was evaluated in a case-control study in Navarre, Spain, in 2010-2012. The cases were 54 children aged 15 months to 10 years with a diagnosis of varicella confirmed by polymerase-chain-reaction. Each case was matched with eight controls by pediatric practice, district of residence and date of birth. The effectiveness was 87% (95% confidence interval: 60% to 97%) for one dose of vaccine and 97% (80% to 100%) for two doses. A single dose was 93% (34% to 100%) effective in the first year, which declined to 61% (95% CI: -64% to 94%) after the third year. In conclusion, varicella vaccine is highly effective in preventing confirmed cases, although this effect declines over time since the first dose. A second dose helps to reestablish very high levels of effectiveness and to reduce the risk of breakthrough varicella.
Keywords: breakthrough varicella; vaccine effectiveness; varicella; varicella vaccine; varicella-zoster virus.