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Doctors look at chicken pox vaccine effectiveness

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CHICAGO, Illinois (Reuters) -- Chicken pox vaccine loses a substantial amount of its effectiveness in the first year after it is administered but overall remains very protective up to eight years, a study said on Tuesday.

The vaccine, which uses a live but weakened virus, was developed in 1974 and has been approved in the United States since 1995. It is recommended for healthy children at 12 to 18 months.

Doctors at the Yale University School of Medicine said a look at 339 children found the effectiveness of the vaccine declines from 99 percent in the first year to 84 percent in years two through eight.

The report, published in this week's Journal of the American Medical Association, rated that level of protection as excellent.

The study was conducted because of reports that chicken pox was occurring among children who had been immunized. The Yale study said such cases found in its study were mild.

The vaccine's effectiveness after one year is also "substantially lower" in children vaccinated before 15 months, the study said, a finding consistent with other reports.

"Changing the age at which immunization ... is begun from 12 to 15 months might alleviate this problem," the report said. "However the improved effectiveness of the vaccine would have to be balanced against both the risk of leaving such children unvaccinated for those three months and the risk that some children might not return for vaccination in a timely manner."

Several vaccines are usually administered at the same time in young children.

"It is clear that the incidence (of chicken pox) in the United States is decreasing as a result of the widespread use of ... vaccine," said the study.

"It is important to monitor closely the incidence (of chicken pox) and the effectiveness of the vaccine over time to determine if a booster dose is needed to improve its effectiveness," it added.

Copyright 2004 Reuters. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

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