YALE (US) — The odds of developing chicken pox are 95 percent lower in children who have received two doses of the vaccine, compared to those who have received only one.
The Centers for Disease Control and Prevention (CDC) began recommending a single dose of chicken pox (varicella) vaccine for children ages 1 to 13 in 1995.
The chicken pox rate fell drastically and studies showed that the effectiveness of one dose was 86 percent. But there was still a high rate of breakthrough illness in immunized children and the CDC changed the immunization policy for chicken pox in 2006, adding a second dose for children ages 4 to 6.
In the current study, reported in the Journal of Infectious Diseases, the effectiveness of two doses is 98.3 percent.
“We weren’t surprised to find that two doses of varicella vaccine are highly effective and are more likely to prevent varicella than a singe dose,” says Eugene Shapiro, professor of pediatrics at Yale University.
“The findings confirm that, at least in the short term, the policy of routinely administering two rather than one dose of varicella vaccine is sensible. Other countries that are routinely immunizing children with varicella vaccine may consider changing to a two-dose regimen.”
Past studies have suggested that two doses of varicella vaccine are linked to higher antibody levels than one dose, but this is the first time the clinical effectiveness of two doses of the vaccine in the general population was assessed.
In a survey of Connecticut children, Shapiro and his team discovered 71 cases of chicken pox in children ages 4 and older. None of these children had received two doses of vaccine; 66 (93 percent) had received one dose and five (7 percent) had received no vaccine.
Because it has only been four years since the CDC policy change, Shapiro also recommends that there be continued monitoring of the effectiveness of two doses to assure that its high degree of effectiveness is sustained.
Researchers from Columbia University contributed to the study.
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