WASHINGTON -- The widely used chicken-pox vaccine seems to reactivate itself when an individual's immunity lessens, possibly creating its own booster mechanism, researchers have discovered.
The vaccine, called Oka, was licensed in 1995 and millions of children receive it every year. It is made from a weakened live virus.
Like all vaccines, Oka provokes an immune response from the patient, preparing the body's defenses to fight in case of infection with the wild form of the virus. When the wild virus later attacks, the body is prepared to respond quickly by boosting the anti-virus material in the bloodstream.
Researchers at the Food and Drug Administration did a four-year study of 4,631 children aged one to 13 who had received the vaccine against the chicken pox virus, known as varicella zoster.
Their findings are reported in the April edition of the journal Nature Medicine.
The vaccine establishes a lifelong latent infection, explained Dr. Dennis N. Klinman of FDA's Center for Biologics Evaluation and Research.
"A latent infection is not like an ongoing illness, it's just that the virus is never completely cleared from the body," he explained.
Because the virus is weakened it doesn't cause disease and no symptoms were reported when it reactivated in the children, he said.
The researchers found 508 cases in which the antibodies in the youngsters' blood increased 400 percent or more from one year to the next.
Such a jump would occur if children were exposed to the wild virus, but the rate of increase is several times higher than would have been expected to occur, the researchers said.
They determined that the increases occurred when the children's resistance to the disease had declined and the latent virus from the vaccine reactivated, causing the body to boost its defensive network.
When the body's resistance is high it keeps both the natural and latent Oka virus in check, Klinman said. When it falls to a lower level the latent virus reactivates. "You don't get sick, but it reactivates your immune system."
The stronger natural form of the chicken pox virus also remains in the body permanently once the symptoms of the disease have passed, usually taking residence in nerve fibers. Years later it can reactivate, particularly in people with reduced immunity, causing a painful skin disease called shingles.
Klinman said no cases of shingles were found in the youths studied.