When her 7-year-old daughter awoke with a fever recently, her torso blanketed by an itchy rash, Trudi Boyd was perplexed. "I thought, 'She's had the vaccine -- this can't possibly be chickenpox.' "

That's exactly what it was, according to the pediatrician who told Boyd her daughter was the third chickenpox patient he'd seen that day -- all of them from St. Mary's Catholic School in Old Town Alexandria.

Since Dec. 9, 11 cases of chickenpox have been diagnosed among students at the school and confirmed by a pediatrician. Eight of these cases involve students who, like Boyd's daughter, had been vaccinated against the disease as babies.

An elementary school in Loudoun County has been hit much harder: 47 cases of chickenpox, half among vaccinated children, have been reported since the end of November, according to Virginia Health Department epidemiologist Denise Sockwell, who declined to identify the school. No recent outbreaks have been reported in the District or suburban Maryland, officials there said.

The phenomenon of what doctors call "breakthrough" chickenpox infections highlights 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 require vaccination for entry to kindergarten or day care, and it is estimated that 75 percent of children under 3 have been innoculated.

But several recent studies, the most recent of which was published Dec. 12 in the 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 A. Gershon of Columbia University, are suggesting that a second dose of chickenpox vaccine, like the one given for measles, be added to augment protection.

It is a suggestion that federal health officials say they are pondering. "We know the vaccine works, especially in preventing severe disease," said Jane F. Seward, chief of childhood vaccine-preventable diseases at the Centers for Disease Control and Prevention (CDC). "We always knew the vaccine wasn't 100 percent effective. There's certainly a consideration of adding a second dose . . . and we would like to understand what places some children at higher risk of a breakthrough infection" while others do not succumb.

Seven studies of the effectiveness of the varicella vaccine, licensed in 1995, have found that the shot is 71 to 100 percent effective in preventing chickenpox of any severity and 95 to 100 percent effective against moderate to severe disease -- the most contagious form, which can result in hospitalization or death. The disease is spread through exposure to an infected person and typically develops 14 to 21 days later.

Officials at Merck, which manufactures the vaccine, said that the New England Journal study differs from earlier reports, which showed that the vaccine was significantly more effective. "Other investigations need to be done," said Richard M. Haupt, senior medical director for pediatric vaccines at Merck. "We will continue to look at this and the CDC will, as well. But the important thing to remember is that you may not be completely protected, but the chances are you'll be protected from severe disease."

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."

The New England Journal report involves a chickenpox outbreak at a Concord, N.H., day care center that affected large numbers of vaccinated children and resulted in an unusually low overall effectiveness rate for the vaccine -- 44 percent. Officials at the CDC and the state health department found that the vaccine was 86 percent effective in preventing severe cases, defined as more than 500 lesions.

The New Hampshire outbreak involved 25 of 88 children who attended the same day care center between Dec. 1, 2000, and Jan. 11, 2001. The first child to contract the disease was an otherwise healthy 4-year-old boy who had been inoculated three years earlier. He subsequently infected more than half his classmates.

Investigators 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

"Although policy cannot be established on the basis of one outbreak," the investigative team led by Karin Galil, formerly of the CDC, wrote, "the findings in this investigation raise concerns that the current vaccination strategy may not protect all children adequately."

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. 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.

In the past seven years the CDC has been called in by local health officials to investigate chickenpox outbreaks in 10 states, including Maryland. An investigation conducted in January 2001 by CDC investigators at a Montgomery County elementary school placed the vaccine's effectiveness rate at 59 percent, according to CDC's Aisha Jumaan, who directed the probe.

The outbreaks in New Hampshire and Loudoun County and the recent cluster of infections in Alexandria appear to be increasingly common, health officials say.

"These are happening all the time, and we don't usually get called," said Jumaan, noting that chickenpox won't become a reportable disease in all states until 2003. Sometimes, she said, parents call the CDC to report cases and CDC officials then call state health departments to alert them to a possible outbreak.

"The issue for us is: Is this a random event or is it really telling us something about the vaccine?" Jumaan 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.

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Even vaccinated kids can get chickenpox.