NEW YORK (Reuters Health) - When it comes to vaccinating children against the chickenpox virus, sooner may not be better, an analysis of a chickenpox outbreak at a Pennsylvania day care center suggests.

Current guidelines call for vaccinating children against chickenpox at between 12 and 18 months of age, but in the day care center outbreak, children who had been vaccinated at 12, 13 or 14 months were three times more likely to develop chickenpox than those vaccinated later, Dr. Karin Galil of the Centers for Disease Control and Prevention in Atlanta, Georgia, told Reuters Health.

"This suggests that delaying the age of vaccination slightly might improve the effectiveness of the vaccine," she said. But she emphasized that this is the first study of the currently licensed chickenpox vaccine to suggest that early vaccinations are less protective.

"It is important to keep in mind that one small study does not provide enough evidence to prove that vaccinating children at less than 14 months of age increases their risk of vaccine failure," according to Galil. But she added, "It does mean that this needs to be studied further in other and larger studies to determine if it increases risk."

A virus called varicella-zoster, which is a member of the herpes family, causes both chickenpox and shingles. For most children, chickenpox is more of a nuisance than a real health threat, but the disease can cause severe complications. Before the varicella vaccine was licensed in 1995, roughly 100 people--50 children and 50 adults--in the US died from chickenpox and about 11,000 individuals were hospitalized with serious complications each year, Galil noted.

The CDC researcher added that the vaccine is "extremely good" at protecting children and adults from severe chickenpox and its complications, but she pointed out that the vaccine does not provide complete protection. In most studies the vaccine protected 70% to 88% of people against any infection with the virus, she said.

When Galil's team analyzed a March 2000 chickenpox outbreak at a Pennsylvania day care center, they found that the varicella vaccine provided substantial protection against chickenpox. About one third of chickenpox cases occurred among children who had been vaccinated, but the vaccine was 79% effective against all cases of chickenpox and 95% effective against moderate to severe chickenpox, according to a report in a recent issue of the Journal of Infectious Diseases.

But among vaccinated children, those who had been vaccinated at 14 months of age or younger had triple the risk of developing chickenpox. Because of the small number of children at the day care center, the increased risk was not statistically significant. And Galil noted that vaccinated children tend to develop a much milder form of chickenpox than unvaccinated children.

Still, the investigators conclude that more study is needed to confirm the findings and to determine the best age for varicella vaccination.

SOURCE: Journal of Infectious Diseases 2002;186:102-105.

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