Study: Kids’ Chickenpox Vaccine Helps Protect Babies Too

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Some parents take their children to “pox parties” to expose them to the chickenpox virus and encourage immunity. Health experts suggest just vaccinating your kids instead.

Not only is the varicella (chickenpox) vaccine safe and effective, but by protecting children who receive the vaccine, it also contributes to “herd immunity,” further safeguarding infants who are too young for the shot, according to a new study published in the journal Pediatrics.

The U.S. initiated a varicella vaccine program in 1995, recommending routine immunization of all children aged 1 year or older. To find out how the subsequent rise in community-wide immunity affected the risk of chickenpox in unvaccinated infants, researchers looked at data from Los Angeles County, Calif., and West Philadelphia from 1995 to 2008.

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Among the 600,000 people included in the surveillance in both sites, about 2% were younger than 12 months. Overall, a total of 11,336 cases of chickenpox were reported, 519 of which occurred in babies younger than 1. The researchers found that as routine varicella vaccination increased in older children, the rate of chickenpox cases in babies under 1 dropped precipitously — by nearly 90%, from 15.6 cases per 1,000 infants in 1995 to 1.6 cases per 1,000 infants in 2008.

Chickenpox is so common a childhood illness that many people don’t realize it can be serious, especially in babies, who tend to be more vulnerable to fever and skin and ear infections and, rarely, potentially fatal complications like pneumonia. (In in the pre-vaccine era, infants were four times more likely to die from chickenpox than children over age 1.) The only way to protect the younger population is to immunize the older one, and prevent the virus from spreading; according to the World Health Organization, about 80% of the population must receive varicella vaccination in order to achieve herd immunity.

Currently, at least 45 states require public school children to be immunized against chickenpox, according to the Centers for Disease Control and Prevention, and childhood varicella vaccination rates top 90% in some states. While major strides have been made against chickenpox, there are still outbreaks — primarily in unvaccinated populations.

Parents are increasingly opting out of many other routine childhood vaccinations as well, in part because of lingering concerns over vaccine safety. As many studies have shown, however, the potential for harm is much greater when parents don’t vaccinate their kids.

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“Improving varicella vaccination coverage in all age groups will further reduce the risk of varicella exposure and protect those not eligible for varicella vaccination,” wrote the study’s authors.

Meredith Melnick is a reporter at TIME. Find her on Twitter at @MeredithCM. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.