Fewer babies are coming down with chicken pox ever since the advent of the vaccine some 15 years ago, a new study finds. While infants under 1 are too young to receive the vaccine, they benefit from being surrounded by children who have received the drug, researchers assert. Officials refer to this benefit to the unvaccinated as “herd” immunity.

The occurrence of chicken pox in infants decreased 90 percent in the period between 1995, when the vaccine first became available, through 2008, according to a report in the journal Pediatrics.

Chicken pox is an extremely contagious childhood virus that causes an itchy rash and fever. In infants, it can lead to pneumonia and be fatal. Prior to 1995, infants were four times more likely to die from chicken pox than children over 1 year old, according to the Centers for Disease Control and Prevention. Last year, more than 100 children died from chicken pox in the U.S.

By getting children vaccinated against chicken pox, “we decrease the amount of disease that’s going around,” said Adriana Lopez, study author and epidemiologist at the CDC. “That, therefore, decreases the exposures that people who aren’t protected would come across.”

But the disease is still circulating, and even more can be done, said pediatrician Dr. Jennifer Shu. “By increasing coverage in those who are recommended to be vaccinated, then we will be able to protect those who cannot get vaccinated, like infants.”