Effectiveness of chickenpox vaccine drops after first year

Because some children vaccinated against chickenpox eventually develop the disease, researchers at Yale University decided to test the immunization’s effectiveness.

They found that protection is strongest in the first year after the varicella vaccine is given -- with a 97% prevention rate -- but that the effectiveness drops to 86% in the second year and 81% in the seventh and eighth years.

The researchers also found that if the varicella vaccine is administered before a child is 15 months old, it doesn’t work as well in the following 12 months. The vaccine, a relatively new addition to the group of childhood vaccinations, was approved in 1995 and is given to youngsters ages 12 to 18 months or to nonimmunized older children.

“The overall effectiveness of 87% is still excellent ... so the bottom line is that overall the vaccine is still quite effective,” said lead researcher Dr. Eugene D. Shapiro, a professor of pediatrics, epidemiology and investigative medicine.

The researchers theorize that the infections in vaccinated youngsters, while generally mild, are the result of waning immunity in some immunized kids and vaccine failure in others.

The study, based on cases of chickenpox diagnosed from March 1997 to June 2003 in 339 children, was published in the current Journal of the American Medical Assn.

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Jane E. Allen