Dr. Karin Galil of the Centers for Disease Control and Prevention (news - web sites) in Atlanta, Georgia and colleagues report their findings in the December 12th issue of The New England Journal of Medicine (news - web sites).

The researchers evaluated an outbreak of chickenpox, which is caused by the varicella virus, at a daycare center in New Hampshire. A total of 88 parents returned a questionnaire that aimed to gauge prior chickenpox illness and vaccination among the children. In all, 25 children came down with chickenpox between December 2000 and January 2001. The researchers sourced the outbreak to a 4-year-old child who had been vaccinated for chickenpox 3 years prior to contracting the illness.

The child infected about half of his classmates who had no prior history of chickenpox infection. At the time of the outbreak, roughly 73% of kids old enough for chickenpox vaccine had received it, the report indicates.

"The effectiveness of the vaccine was 44% against disease of any severity and 86% against moderate or severe disease," write Galil and colleagues. Experts have estimated that the chickenpox vaccine is between 71% to 100% effective at preventing varicella infection.

Children who had been vaccinated 3 years or more before the outbreak were at greater risk of vaccination failure than those who had been vaccinated more recently, they add.

On the surface it appears that immunity against chickenpox weakened as time passed after vaccination. However, the authors note that "the reasons for the poor performance of the vaccine are not apparent.

"Although policy cannot be established on the basis of one outbreak, the findings in this investigation raise concern that the current vaccination strategy may not protect all children adequately," the authors write.

Nonetheless, the investigators point out that the illness is much less of a threat today than it was before the era of chickenpox vaccination, when there were roughly 11,000 hospitalizations and 100 deaths from the disease annually.

"Vaccination remains the most effective strategy for protecting children and adults against illness and death due to varicella," Galil and colleagues conclude.

Current guidelines call for one dose of chickenpox vaccine for children between the ages of 1 and 12 years and two doses of vaccine for people over 13.

"It has long been known...that 'breakthrough' varicella may nevertheless develop in 10 to 15 percent of vaccinated persons," Dr. Anne A. Gershon of Columbia University in New York City writes in an accompanying editorial.

Gershon suggests that a second dose of chickenpox vaccine "should decrease the number of children who have...vaccine failure and might also prevent waning immunity, if it does indeed currently occur."

What's more, Gershon points out that it eventually took the routine administration of two doses of measles vaccine to control measles in the US.

"The time for exploring the possibility of routinely administering two doses of varicella vaccine to children seems to have arrived," Gershon concludes.