Medicine. Cheeseburgers. The speed limit. There are many things in this world that doubling up on is a bad idea.

However, when it comes to varicella, or chicken pox, vaccine, researchers at Yale School of Medicine have found two doses are better than one.

In fact, the odds of developing chicken pox were 95 percent lower in children who had received two doses of the vaccine compared with those who had received only one dose.

Published in the February 1 issue of Journal of Infectious Diseases, the study was led by Eugene D. Shapiro, M.D., professor in the Department of Pediatrics at Yale and his colleagues at Yale and Columbia universities.

Dosage dispute

The Centers for Disease Control and Prevention (CDC) began recommending a single dose of chicken pox vaccine for children ages 1 to 13 in 1995. The chicken pox rate fell drastically and studies showed the effectiveness of one dose was 86 percent.

But there was still a high rate of breakthrough illness in immunized children.

The CDC then changed the immunization policy for chicken pox in 2006, adding a second dose for children ages 4 to 6.

In this study, Shapiro and his team showed the effectiveness of two doses is 98.3 percent.

Past studies have suggested that two doses of varicella vaccine are linked to higher antibody levels than one dose, but this is the first study to assess the clinical effectiveness of two doses of the vaccine in the general population.

The case for double doses

In a survey of Connecticut children, Shapiro and his team discovered 71 cases of chicken pox in children ages four 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.

"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," said Shapiro. "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."

Shapiro said the findings also might inspire other countries that are routinely immunizing children against chicken pox to consider changing to a two-dose regimen.

Because it has only been four years since the CDC policy change, Shapiro also recommends there be continued monitoring of the effectiveness of two doses to assure that its high degree of effectiveness is sustained.