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Varicella vaccine affords good but not perfect protection against varicella; post-licensing studies have estimated its efficacy in the range of 80%–85%. A large study monitoring disease activity in a single California community now provides evidence that the vaccine's protective efficacy wanes further with time.
In the 10 years after varicella vaccine was introduced in 1995, cases of varicella plummeted in Antelope Valley, California (population 350,000), from 2794 in 1995 to an annual average of 420 in 2003–2004. Almost 10% of cases occurred in subjects vaccinated at least 6 weeks previously (breakthrough varicella), with the proportion of breakthrough cases rising from 1% in 1996 to 18% in 2000 and 60% in 2004.
Among children vaccinated bet…