Chicken pox rates decreased following vaccinations of older children

A new study shows that fewer babies were infected with chicken pox after the U.S. started to routinely vaccinate older children in 1995.

Infants under the age of 12 months are too young to get the chicken pox vaccine but are largely protected against the illness if their older siblings and daycare playmates are vaccinated. Herd Immunity, the idea that if enough people are vaccinated a disease will not be able to be spread, appears to be the reason, according to Reuters.

"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," Dr. Eugene Shapiro, who studies infectious diseases in children at Yale School of Medicine, said, Reuters reports. "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."

In 1995, the U.S. began a rigorous program to vaccinate all children against the chicken pox, which usually only causes an itchy rash for a few days but can lead to more serious complications. Having chicken pox as a child can increase the risk that someone will develop shingles later in life because they are caused by the same virus.

Researchers from the U.S. Centers for Disease Control and Prevention tracked the number of chicken pox infections in babies from the start of the program through 2008. The study included 600,000 from two communities in Los Angeles and Philadelphia, Reuters reports.

The researchers found that as the number of one, two and three year olds who were vaccinated against chicken pox increased, the number of babies with the infection decreased. Initially, 16 out of every 1,000 infants acquired the illness. By 2008, that number fell to two out of every 1,000.

"It makes sense, it's not surprising," Shapiro said, according to Reuters. "The point is that the incidence of chicken pox has gone down dramatically. If there's less chicken pox around, if you're susceptible, you're less likely to become exposed."