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Chicken pox vaccine's staying power questioned.
A disease outbreak in a New Hampshire day-care center may indicate the need for periodic boosters.

By Victoria Stagg Elliott, AMNews staff. Jan. 20, 2003. Additional information

The 4-year-old boy was healthy and had received all of his shots, including the one for varicella three years before. But one morning at a day-care center in a small town near Concord, N.H., his body erupted in a rash and he was sent home.

The boy had chicken pox and had infected more than a dozen of his classmates with the illness, even though most had also received the vaccine. Within two months, another dozen would also be diagnosed with what was, until the 1995 licensure of the varicella vaccine, one of the leading causes of morbidity among children.

This was not supposed to happen, according to a case study published in the New England Journal of Medicine last month.

"They were so healthy as a group," said Karin Galil, MD, MPH, the lead researcher who investigated the outbreak while a medical epidemiologist at the Centers for Disease Control and Prevention. "And a lot of the risk factors associated with vaccine failure, such as asthma, just weren't present."

Dr. Galil and her fellow CDC investigators found the vaccine still prevented the most severe forms of chicken pox, but that children who had received it more than three years earlier were most at risk for developing mild or moderate forms of the disease.

An editorial accompanying the article suggests considering whether a booster shot may be needed a few years after the initial dose.

The chicken pox vaccine was approved in 1995.

"This outbreak constitutes a warning signal," wrote Anne A. Gershon, MD, director of the infectious disease division at Columbia University College of Physicians and Surgeons in New York. " The time for exploring the possibility of routinely administering two doses of varicella vaccine to children seems to have arrived."

Infectious disease experts and the authors of the paper say, however, that it is far too soon to recommend additional doses. Other studies have not found waning efficacy, although time will tell.

Although first approved in 1995, it took a couple of years for kids to receive the vaccine in great numbers and, consequently, for the disease's mortality and morbidity to decline.

If vaccine efficacy really does wane, more evidence will surface in the next few years.

"It's an important study, but one study does not policy make," said Harry Keyserling, MD, professor of pediatrics at Emory University School of Medicine in Atlanta. "We might have to wait another three or four years until we're dealing with a highly immunized cohort of first-graders to determine if a booster is needed."

But experts also suggest that there may be other reasons for this outbreak. The vaccine, which must be kept frozen, may have been mishandled. And the kids may not have received it at the ideal time.

"This study may be an indication that we need an extra dose," said Tina Tan, MD, an infectious disease specialist at Children's Memorial Hospital in Chicago. "It may also just be one of these fluke situations."

Critics of the study also say that the method of using parents to report illnesses rather than laboratories to confirm them leaves the question open as to whether the children really had chicken pox or a rash from some other cause. But, experts said that if a booster was needed, parents would be amenable to it, especially because a vaccine combining measles, mumps, rubella and varicella is expected in the next couple of years.

"This would improve immunization coverage, as well as making it easier on the primary care physician, family and most importantly the child," said Alan Shapiro, MD, medical director of the South Bronx Health Center for Children and Families in New York.

A spokeswoman for Merck & Co. Inc, the vaccine's manufacturer, said they were looking at the study to determine its implications.

"We agree the article should be taken seriously but it does diverge from previously published studies," said Kelley Dougherty, company spokeswoman.

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ADDITIONAL INFORMATION:
Follow-up on a rash of pox
Objective: Determine the cause of a chicken pox outbreak among a group of vaccinated children at a day-care center in a small town in New Hampshire.
Method: Parents, physicians and other health care professionals filled out questionnaires about the children's medical and vaccination history.
Results: Varicella developed in 25 of the 88 children attending the day-care center over a six-week period. The vaccine provided 44% protection against any form of chicken pox and 86% protection against moderate and severe forms of the disease. Children who had received the vaccine more than three years earlier were at higher risk of developing chicken pox.
Conclusion: A longer interval since vaccination is associated with a higher rate of vaccine failure, although it still provides good protection against more severe forms of the disease.

Source: New England Journal of Medicine, Dec. 12, 2002