In that group of children, 2,826 also received a booster dose of varicella vaccine between 2006 and 2009.
Slightly more than 1,500 cases of breakthrough cases of chickenpox occurred during the study period. All of the cases occurred after the first dose of the vaccine. No breakthrough cases were reported after the second dose of vaccine, the study found.
Only 2 percent of the breakthrough cases were severe. Overall, the rate of chickenpox in vaccinated children was about 10 times lower than it would have been had they not been vaccinated, according to the study.
Shingles is unusual in children, but researchers estimated the children who were vaccinated had a 40 percent reduced risk of shingles.
Because breakthrough cases occurred soon after vaccination, it's possible that the second dose would be more effective if given earlier than the recommended age of 4 to 6 years, the authors noted.
Dr. Kenneth Bromberg, director of the Vaccine Research Center at the Brooklyn Hospital Center in New York City, commented favorably on the study.
"These researchers answered important questions about efficacy with one dose; it's about 90 percent, and the effectiveness of two doses seems to be about 100 percent," Bromberg said.
Both experts said it looks like this vaccine will continue to protect these children into adulthood, although no studies have yet confirmed these findings because the vaccine has only been available for 17 years.
With most vaccines, effectiveness wanes a little bit over time, said Baxter. "But with the varicella vaccine, it got more and more effective over time," he noted.
Will adults need another dose of vaccine? From this study, Bromberg said, "it looks promising that the answer will be no."
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