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Chickenpox vaccine fails at US day care center

By Anne Harding

CHICAGO, Dec 19 (Reuters Health) - A new report reveals that, at least among one group of youngsters, chickenpox vaccine was much less effective than past research has demonstrated.

But the findings don't mean parents should stop vaccinating their children against chickenpox, one of the researchers, Jane Seward of the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, told Reuters Health.

While some may consider chickenpox a mild disease, Seward noted that before the vaccine came into use in 1995, the viral illness killed 100 people and sent 10,000 to the hospital each year in the US. Complications of chickenpox can be serious, including bacterial infections in various parts of the body such as the lungs, as well as brain infections.

"You can't predict who is going to have that bad outcome," Seward said.

And, she noted, the number of severe cases of chickenpox has fallen by 85% since the introduction of the vaccine.

Seward and her colleagues reported Tuesday on their investigation of a recent outbreak of chickenpox at a New Hampshire day care center. They presented their findings here at the Interscience Conference on Antimicrobial Agents and Chemotherapy.

Past studies have shown the effectiveness of the vaccine against varicella, the virus that causes chickenpox, ranges from 71% to 91%. The current study found, however, that it was only about 40% effective.

This is the first research that has shown "anything significantly below" the vaccine effectiveness of earlier reports, co-investigator B. R. Lee of the CDC told Reuters Health.

The outbreak in 23 children also began with a child who had been vaccinated, contradicting the belief that such "breakthrough" cases are not contagious, Seward noted. The child, a 4-year-old, was confirmed not to have caught chickenpox from the vaccine, but probably contracted it from a sibling with shingles--which is caused by the same virus.

Seward and Lee said they cannot yet explain why the vaccine may have been ineffective in this group of youngsters. "We'd like to really understand what factors came together to produce it," Seward added. "We're not dismissing it."

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