WASHINGTON -- When her 7-year-old daughter recently awoke with a fever and an itchy rash blanketing her torso, Trudi Boyd was perplexed.

"I thought, ‘She's had the vaccine -- this can't possibly be chickenpox.' "

But it was chickenpox, according to the pediatrician. Boyd's daughter was the third chickenpox patient the doctor had treated that day -- all of them from St. Mary's Catholic School in Old Town Alexandria, Va.

Since about mid-December, 11 cases of chickenpox have been diagnosed and confirmed among students at the school. Eight of the cases, called "breakthrough" chickenpox infections, involved students who, like Boyd's daughter, had been vaccinated against the disease as babies.

The disease is spread through exposure to an infected person and typically develops 14 to 21 days later.

In West Virginia, recent outbreaks have occurred in Berkeley, Marshall and Ohio counties, said Dr. Danae Bixler, infectious disease epidemiologist with the state Bureau of Public Health.

"We've had several outbreaks in the state over the last few months," Bixler said. "There have been a number of previously vaccinated children who have gotten the disease."

In the past seven years, local health officials have called in the CDC to investigate chickenpox outbreaks in a total of 10 states.

These breakthrough infections highlight what some health officials say may be a limitation of the varicella vaccine, administered as a single shot to babies between the ages of 12 and 18 months.

Most states -- not West Virginia -- require vaccination for entry to kindergarten or day care. It is estimated that 75 percent of children under age 3 have been inoculated.

But several studies, the most recent in the Dec. 12 New England Journal of Medicine, suggest that the single-shot regimen may be insufficient to prevent outbreaks of chickenpox among vaccinated children.

Some vaccine experts, among them Anne Gershon of Columbia University, are suggesting that a second dose of chickenpox vaccine, like the one given for measles, be added to augment protection.

Federal health officials say they are pondering the suggestion. They say that while the vaccine works, especially by preventing the more severe cases of chickenpox, they knew it was not 100 percent effective.

Bixler agrees.

"Early data suggest around an 85 percent efficacy rate with the vaccine," she said. "So there's a 15 percent risk of getting it even after the vaccine.

"But what we expect is 100 percent efficacy in severe disease."

Seven studies of the effectiveness of the varicella vaccine, licensed in 1995, have found 95 to 100 percent effectiveness against moderate to severe disease -- the most contagious form, which can result in hospitalization or death.

Bixler said the vaccine first was tested in children with leukemia, resulting in drastic improvements in both disease severity and mortality from chickenpox.

Before the advent of the vaccine, 11,000 Americans were hospitalized annually with complications from chickenpox and 100 of them died. The vaccine has led to an 80 percent drop in both complications and deaths, health officials said.

Although people with impaired immune systems are most at risk from the virus, some deaths have occurred in otherwise healthy children who developed pneumonia, toxic shock syndrome or necrotizing fasciitis, more commonly known as "flesh-eating bacteria."

Researchers studying a Concord, N.H., day-care center found that children who had been vaccinated three or more years before the outbreak were twice as likely to contract chickenpox as those inoculated more recently. Of the 25 children affected, 17 had been vaccinated. These children all had mild cases and fewer than 50 lesions.

Investigators did not find that the outbreak was caused by an improper dose of vaccine or by a problem with the way it had been stored or administered

The New Hampshire outbreak is considered by epidemiologists to be an "outlier" because the effectiveness rate is so much lower than has been found in previous studies. This case alone will not lead to a change in immunization policy, CDC officials said.

There is not much parents can do to prevent infections in vaccinated children. Many schools require that children with chickenpox stay out of school until all lesions crust over, a process that can take about five days. Because chickenpox lesions resemble insect bites, mild cases may be overlooked or misdiagnosed.

The decision of whether to add a booster probably won't be made until after experts examine data from a long-term study of 90,000 children.

Meanwhile, Bixler said even a single chickenpox vaccine helps immensely in saving children from sickness and death.