WEDNESDAY, March 14, 2007 (HealthDay News) -- Protection against chickenpox slowly fades in children immunized against the disease, suggests a study that adds support to a recommendation for a booster shot of the vaccine.
The study found the incidence of chicken pox increased over time among vaccinated children -- from 1.6 cases per person-year one year after immunization to 9 cases per person-year five years later and more than 58 cases per person-year nine years later, according to the report.
"This is the kind of monitoring we do to check a vaccination program," said senior researcher Dr. Jane F. Seward, acting deputy director of the U.S. Centers for Disease Control and Prevention's division of viral diseases. "This is the first time monitoring shows an increase in the number of cases over time."
Seward's team published the findings in the March 15 issue of the New England Journal of Medicine.
A vaccine against varicella, the medical name for chickenpox, was introduced in 1995 with a recommendation that it be given during the first year of life. Last year, a recommendation for a second shot at 4 to 6 years of age was added by the American Academy of Pediatrics and other medical bodies.
"No vaccine we have is 100 percent perfect," explained Dr. Robert Frenck, a professor of pediatrics in infectious diseases at Cincinnati Children's Hospital and a member of the American Academy of Pediatrics committee for infectious diseases. "You have people who don't respond or lose immunity over time."
Chickenpox is rarely fatal, but it can cause an outbreak of hundreds of temporarily disfiguring open sores. The cases found in the study -- which focused on immunized children in Antelope Valley, Calif., northeast of Los Angeles -- tended to be relatively mild, Seward said.
"In general, the cases were very modified from natural varicella," she said. "So, the vaccine does appear to stand up."
However, the study also shows that "there is a greater chance of the disease not being mild over time," Seward said. "We need to keep following this group to see if it translates into more disease over time."
"The important thing is that the [first-shot] protection is about 90 percent effective," Frenck said. "There was an 85 percent reduction in cases of varicella over 10 years."
A new multiple vaccine, approved last year, should make it easier for children to get both the first and recommended second shot, Frenck said. Protection against chicken pox has been added to the MMR vaccine, aimed at measles, mumps and rubella (German measles).
A second shot offers advantages in addition to protection against a childhood outbreak, Seward said, since chickenpox can occur in adults. "There is definitely improved immune response after the second dose. There is better immunity in the long term," she said.
There's more on the chickenpox vaccine at the U.S. National Network for Immunization Information.