Even though babies under one year old are too young to get the chicken pox vaccine, far fewer of them got the itchy rash after the U.S. started routinely vaccinating older children in 1995, according to a new study.

Researchers said that infants are now largely protected against chicken pox if their older siblings and daycare peers have been vaccinated, due to “herd immunity” - the idea that if enough people are immune to a disease, it won’t be able to spread.

“It is a measure of the success of the (vaccination) program and the fact that we should encourage all parents to fully vaccinate their children,” said Dr. Eugene Shapiro, who studies infectious diseases in kids at the Yale School of Medicine in New Haven.

“If all children got vaccinated, then the small proportion who are still developing chicken pox would become even smaller, and so the risk to these infants who aren’t yet vaccinated would be even lower,” added Shapiro, who wasn’t involved in the new study.

Sixteen years ago, the U.S. began a program to vaccinate all kids against chicken pox - which usually just leaves them itchy for a few days but can cause serious complications, especially in pregnant women and babies.

Having chicken pox as a kid also increases the risk that someone will get shingles, which is caused by the same virus, when they’re older.

Now, the Centers for Disease Control and Prevention recommends that all kids between 12 months and 12 years of age - and older kids and adults who haven’t yet been vaccinated or had chicken pox - should get two doses of the vaccine, and many states require that kids be immunized before starting school.

Babies aren’t vaccinated because many acquire some of their mom’s immune defenses against chicken pox, and the vaccine won’t work until those have faded with age. The vaccine’s safety also hasn’t been tested in infants.

Researchers from the CDC tracked the number of chicken pox cases in babies under one year old from the start of the vaccination program through 2008 in two communities in Los Angeles County and West Philadelphia, including more than 600,000 people.

They found that as the number of one-, two- and three-year-olds vaccinated against chicken pox increased, the number of cases in babies fell - from 16 out of every 1,000 infants in 1995 to fewer than two in every 1,000 by 2008.

All in all, the number of new infections in the youngest age group dropped almost 90 percent during the study, researchers reported in Pediatrics on Monday.

“During the pre-vaccine era…infants were at big risk of severe disease,” said Adriana Lopez, from the CDC, who worked on the study.

The finding, she told Reuters Health, “shows that the impact of this vaccination program has been great, in terms of not only decreasing disease in those who have been vaccinated, but protecting those who can’t get vaccinated.”

“It makes sense, it’s not surprising,” Shapiro told Reuters Health. “The point is that the incidence of chicken pox has gone down dramatically.”

Even in infants who are too young to get the vaccine, he added, “If there’s less chicken pox around, if you’re susceptible, you’re less likely to become exposed.”

Shapiro said that parents should still take the proper precautions, including going to see their pediatrician, if their baby is exposed to someone who has chicken pox. In some cases, doctors can give those infants a high dose of antibodies targeted at the chicken pox virus to prevent them from getting sick.

Lopez added that moms and dads shouldn’t get complacent about vaccinating just because chicken pox is much less common that it used to be.

“We still see disease circulating,” she said. “Even though the number of cases has continued to go down, it’s still important to have those who can get vaccinated, get vaccinated.”

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SOURCE: Pediatrics, online November 28, 2011.