BOSTON Merck's chickenpox vaccine Varivax not only loses its effectiveness after a while, but it has also changed the profile of the disease in the population, U.S. researchers reported on Wednesday.
The study confirmed what doctors widely knew -- that the vaccine's protection does not last long.
And with fewer natural cases of the disease going around, unvaccinated children or children in whom the first dose of the vaccine fails to work have been catching the highly contagious disease later in life, when the risk of severe complications is greater, they said.
"If you're unvaccinated and you get it later in life, there's a 20-times greater risk of dying compared to a child, and a 10 to 15 times greater chance of getting hospitalized," said Jane Seward of the Centers for Disease Control and Prevention in Atlanta, who worked on the study.
The findings, reported fully for the first time in Thursday's New England Journal of Medicine, have already had an impact.
They helped prompt the CDC's Advisory Committee on Immunization Practices to recommend a booster shot between the ages of 4 and 6. The panel also said in its June 2006 report that children, adolescents and adults should be given boosters as well.
No one knows how long the effects of a second shot will last, said the research team, led by Sandra Chaves of the CDC.
The United States has been vaccinating against chickenpox, also known as varicella, since 1995. But tests have show that the vaccine is not very effective in 15 to 20 percent of children who only receive one dose.
A second dose will certainly provide extra protection, but it is not clear how much. "Instead of 80 to 85 percent efficacy, we're hoping instead to see 90 to 95 percent for the second dose," said Seward, acting deputy director of the CDC's division of viral diseases.
The Chaves team used vaccination and illness data from Antelope Valley, California, northeast of Los Angeles, to track Varivax's effectiveness.
The shots cut the number of cases by 85 percent between 1995 and 2004. In 1995 just 1 percent of the 2,794 reported cases were among children who had been vaccinated. In 2004, there were far fewer cases -- 420 -- but 60 percent were in vaccinated children.
While 73 percent of the youngsters who became ill in 1995 were under age 7, the rate dropped to 30 percent by 2004 because the children who got chickenpox tended to get it at an older age.
And when vaccinated children were infected, they tended to be sicker, probably because they were older.
"Children between the ages of 8 and 12 years who had been vaccinated 5 years or more previously were two times as likely to have moderate-to-severe breakthrough disease as were those who had been vaccinated less than 5 years previously," the researchers wrote.
Last May another vaccine made by Merck to act as a booster for adults, Zostavax, was approved.
The chickenpox virus remains in the body for life and can be reactivated as shingles, a rash that can cause pain that persists for years.
Chickenpox is often considered a harmless childhood disease, but before vaccination, 100 children died each year from complications of chickenpox in the United States alone.