Health officials say a New Hampshire boy immunized against chicken pox passed the infection on to 17 of his day-care classmates who also had received the shot.
No one died or required hospitalization in the 2000-2001 outbreak, which sickened a total of 25 children, though eight had moderate to severe symptoms.
Studies have pegged the chicken pox vaccine's efficacy rate at between 71 percent and 100 percent, and it performs even better at preventing serious bouts of the disease.
Yet in the day-care outbreak, it worked only 44 percent of the time. Those children vaccinated three or more years before the exposure were 2.6 times more likely than those who received the shot more recently to become infected with chicken pox.
This suggests the inoculation's protection might fade with time.
Still, infection experts term such incidents an "outlier," and say the vaccine's real-world performance is strong.
"We have very good surveillance data showing an 80 percent decrease in the incidence of chicken pox in children, and a similar decrease in hospitalizations," says Dr. Karin Galil, a former epidemiologist at the U.S. Centers for Disease Control and Prevention and a co-author of the study. "Overall, the vaccine is working much better than 44 percent."
Dr. Jane Seward, a CDC chicken pox expert, calls the outbreak "a concern." However, Seward says the times vaccines fail are much more visible than when they work as advertised. "We only investigate outbreaks when there are cases."
A report on the vaccine-dodging outbreak appears in tomorrow's issue of The New England Journal of Medicine.
The chicken pox vaccine won U.S. Food and Drug Administration approval in 1995. The inoculation relies on a live but weakened virus to stimulate an immune response against varicella, the virus that causes the disease.
Some parents have been hesitant to have their children immunized against chicken pox, since almost everyone catches the virus early in life with no serious consequences. Yet the disease can be deadly, especially in adults. And the risk of severe infection is considered greater with the wild virus than with the vaccine.
Before the availability of the shot -- which is given once to young children and twice to those aged 13 or older -- chicken pox caused about 11,000 hospitalizations and 100 deaths each year in the United States. Recent studies of children in Philadelphia and around Los Angeles have found that the vaccine has generated 80 percent declines in hospitalizations from the disease.
All but a handful of states now require proof of varicella vaccination for children entering school or day care.
Although experts defend the chicken pox vaccine, they say the New Hampshire outbreak may signal the need for booster shots with the inoculation.
Galil, who is now medical director of Cubist Pharmaceuticals, in Lexington, Mass., says no other studies have shown a similar waning of immunity in children who've received the injection.
However, she adds, when the measles vaccine was first introduced, it also relied on a single injection. After research showed that a booster shot improved protection, a second inoculation was added.
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