The chickenpox vaccine's effectiveness may fade with time, so children should get a second dose when they're 4-6 years old.

That news appears in The New England Journal of Medicine.

Make no mistake: The researchers aren't knocking the vaccine. In fact, they say it has "substantially reduced" chickenpox since becoming recommended for all U.S. children in 1995.

But a single dose of the vaccine may not confer lifelong protection, so a second dose could help, note the researchers, who work at the CDC and the Los Angeles County Department of Health Services.

That finding is in line with the CDC's recommendation that children who have never had chickenpox get two doses of the chickenpox vaccine.

The first dose should be given when kids are 12-15 months old and the second dose when children are 4-6 years old. Children or teens who aren't fully vaccinated can get catch-up shots; talk to your doctor about their vaccine schedule.

Vaccine Study

The new study was based in a community of 350,000 people living in Antelope Valley, Calif., which is about 40 miles northeast of Los Angeles.

The researchers included the CDC's Sandra Chaves, MD, MSc. They tracked reported cases of chickenpox in the community from 1995 through 2004.

At the time, only one dose of the chickenpox vaccine was recommended; a CDC advisory committee recommended the second dose in June 2006.

From 1995 to 2004, a total of 11,356 people in the community were reported to have chickenpox. Most of chickenpox patients -- all but 1,080 people -- had never received the chickenpox vaccine.

Children 8-12 years old who had gotten the chickenpox vaccine at least five years earlier were twice as likely to get moderate-to-severe chickenpox as those who had been vaccinated more recently.

The study provides evidence that the chickenpox vaccine wanes over time, the researchers note. They say the CDC considered their findings in recommending the second dose of the vaccine.

It's not yet clear how long immunity from the vaccine's second dose lasts, note the researchers.