Two doses of the chickenpox vaccine more effectively protect kids from the virus than just one dose, according to a new study.

One dose of the vaccine prevented chickenpox 86 percent of the time, while two doses prevented chickenpox 98.3 percent of the time, in a study of 140 children who received either one or two doses of the vaccine, the researchers said.

This study is the first to show independent clinical evidence that two doses of the vaccine are more effective than one, said study researcher Dr. Eugene Shapiro, professor of pediatrics at Yale University. The Centers for Disease Control and Prevention has recommended two doses of the vaccine for children ages 4 to 6 since 2006.

"Previous studies only looked at antibody responses, but two doses hadn't been shown to prevent the disease more effectively," Shapiro told MyHealthNewsDaily.

Shapiro and his colleagues also examined 71 cases of children with chickenpox in Connecticut. They found that of those children, none of them had received two doses of the vaccine, 66 children (93 percent) received one dose and five children (7 percent) hadn't received the vaccine at all.

The CDC began recommending one dose of the vaccine for children ages 1 to 13 in 1995. In 2006, the agency raised the recommended number of doses to two for kids ages 4 to 6, because of the high frequency of outbreaks in day care centers and schools, he said.

The United States is one of the only countries to recommend two doses of chickenpox vaccine; most recommend one. But the findings should encourage other countries to change their recommendations, Shapiro said.

Although previous studies by the CDC and the vaccine's manufacturer had demonstrated that the two-dose regimen triggers kids' immune systems to produce more antibodies against the virus than one dose, this new study is an independent verification of those results, said Patrick Schlievert, a professor of microbiology at the University of Minnesota who was not involved in the study.

"This is not surprising, but it's really cool that it is so highly effective," Schlievert told MyHealthNewsDaily.

The study will be published in the Feb. 1 issue of the Journal of Infectious Diseases.

Pass it on: Two doses of chickenpox vaccine give more protection than one dose.

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