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Dec. 11

— By Gene Emery

BOSTON (Reuters) - Doctors may need to give two doses of the chickenpox vaccine instead of one, experts said on Wednesday after a study showed a single dose of the vaccine has a far higher failure rate than previously thought.

Karin Galil of the U.S. Centers for Disease Control and Prevention calculated that the vaccine failed to protect 56 percent of children who got the shot. In previous studies, the failure rate ranged from 0 to 29 percent.

Galil's study found that chickenpox spread rapidly through vaccinated children in a New Hampshire day-care center.

"This outbreak constitutes a warning signal," Anne Gershon of the Columbia University College of Physicians and Surgeons in New York wrote in a commentary in the New England Journal of Medicine, which also published the CDC study.

She said a second shot may be necessary to provide meaningful protection, although Gershon noted that studies were needed to see if the cost was worth the benefit.

"The time for exploring the possibility of routinely administering two doses of varicella (chickenpox) vaccine to children seems to have arrived," she said.

Chickenpox causes headache, fever and, eventually, a rash that leads to itchy red spots that are filled with fluid before they crust over and disappear.

While parents often view it as a benign disease, chickenpox can lead to pneumonia, inflammation of the brain, and skin infections.

Before the vaccine was licensed in 1995, chickenpox sent 11,000 people to the hospital each year and killed 100. Half of the fatalities were children.

Since then, the number of hospitalizations for chickenpox has plummeted by 80 percent, according to the CDC researchers.

Chickenpox spreads easily through the air and a child can be contagious three days before symptoms appear.

About 75 percent of U.S. children have received the chickenpox vaccine. It is required for school-age children in at least 26 states and four others -- Louisiana, New Hampshire, Nevada and New York -- will require it next year.

It is not unusual for children to get multiple vaccines -- it takes two doses of measles vaccine to control that disease, for instance, and several doses for polio, hepatitis and other viruses.

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