When the vaccine against chickenpox was introduced in the United States in 1995, medical experts hoped it would be a one-shot deal, with a single injection about the time of a child's first birthday giving lifelong immunity.

Now, experts are learning that's not so. They've found that the protection from a single immunization fades over time, and that a booster shot is needed if you want your child to have the best chance at lifelong protection from chickenpox, or what doctors call varicella.

The experts reached that conclusion after examining 10 years of data and the effects of the vaccine on more than 11,000 people who got the shot during the years 1995 to 2004. Of those individuals, 9.5 percent, or 1,080, experienced so-called "breakthrough disease," researchers reported last year in the New England Journal of Medicine.

"The breakthrough was mild," said Dr. Robert Frenck Jr., a professor of pediatrics at Cincinnati Children's Hospital and a member of the American Academy of Pediatrics' committee on infectious diseases.

Chickenpox, which is caused by an infection by the varicella-zoster virus, typically starts as a rash on the face that spreads. The rash begins as red bumps that become blisters. Often a child can get hundreds of blisters, which crust over and fall off the body in a week or two.

The children who suffered "breakthrough" chickenpox typically got fewer lesions, Frenck said, perhaps 20 to 50. Still, schools and day-care centers would not welcome them back until the sores had dried or crusted, because the disease can spread easily from person to person.

Based on those findings, experts from the U.S. Centers for Disease Control and Prevention and elsewhere are now recommending that a second booster shot be given to children between 4 and 6 years of age.

There is some leeway in that schedule, said Dr. Anne Gershon, professor of pediatrics at Columbia University College of Physicians and Surgeons, in New York City.

"The booster can be given if (it has been) at least four weeks after the first dose," said Gershon, who is also president-elect of the Infectious Diseases Society of America. Or, it can be given years later, if necessary, she said.

According to the CDC, all children ages 12 months through 12 years old should have two doses of the vaccine. The first dose can be given at 12 to 15 months of age, and the second at 4 to 6 years. Older children, 13 and up, who haven't had the disease in childhood, can be given two doses four to eight weeks apart.

Some adults should consider the vaccine. Ask your health-care provider if you fall into that recommended group, which includes people who have never had the disease.

No vaccine is perfect, experts said.

In 2006, a combination vaccine that included not only chickenpox but measles, mumps, and rubella was licensed, cutting down on the number of shots needed. But that combination vaccine is not currently available in the United States, said Nalini Saligram, a spokeswoman for Merck & Co., which makes it. But it is expected to be back in supply by 2009.

Learn more: American Academy of Family Physicians, with information on the chickenpox vaccine

http://familydoctor.org/online/famdocen/home/healthy/vaccines/193.html