BOSTON, March 14 (Reuters) — The chickenpox vaccine Varivax has changed the profile of the disease in the population, researchers are reporting.
In a study appearing Thursday in The New England Journal of Medicine, the researchers confirm what doctors have already known — that the vaccine has sharply reduced the number of cases in children but that its protection does not last long.
With fewer natural cases of the disease, the study says, unvaccinated children or those whose first dose of the vaccine fails to work are getting chickenpox later in life, when the risk of complications is higher.
“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 Dr. Jane Seward of the Centers for Disease Control and Prevention in Atlanta, who worked on the study.
Preliminary findings have already helped prompt the agency’s Advisory Committee on Immunization Practices to recommend a booster shot at age 4 to 6, and also for older children, adolescents and adults.
No one knows how long the effects of a second shot would last, said the research team, led by Dr. Sandra Chaves of the C.D.C.
The United States has been vaccinating against chickenpox since 1995. But tests have shown that the vaccine is not very effective in 15 percent to 20 percent of children who receive only one dose.
A second dose would 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 Dr. Seward, the acting deputy director of the centers’ division of viral diseases.
Dr. Chaves’s team used vaccination and illness data from Antelope Valley, Calif., northeast of Los Angeles, to track the effectiveness of Varivax, which is made by Merck.
The shots cut the number of cases by 85 percent between 1995 and 2004. In 1995, 1 percent of the 2,794 reported cases were among vaccinated children. In 2004, there were 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 were older.
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 five 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 five years previously,” the researchers wrote.
Last May, another vaccine, Zostavax, also made by Merck, was approved as a booster for adults.
The chickenpox virus remains in the body for life and can be reactivated as shingles, a painful rash.
