Health organizations recommend children receive the varicella vaccine at one year old to protect them against chickenpox, but the vaccine appears to have another benefit: it cuts the risk of shingles, a painful and potentially debilitating rash caused by the reactivated chickenpox virus, by more than half in children over two years old, according to a new study.

Approximately 38 per 100,000 children vaccinated against chickenpox developed shingles per year, compared with 170 per 100,000 unvaccinated children, researchers found. Furthermore, shingles infection rates were lower in children who received both recommended doses of the chickenpox vaccine compared with those who only got the first dose.

Chickenpox, a once common childhood virus that causes fever and a rash lasting up to a week, rarely causes death in children. Before the vaccine, two to three out of every 1,000 U.S. children who got the disease were hospitalized, and approximately 100 children a year died from it. It is often more severe in adolescents and adults.

Serious complications such as infection and brain inflammation can occur, as well as permanent scarring, but the bigger threat from chickenpox is what can happen years later. After an infection, the varicella virus remains latent in nerve roots and can reactivate to cause shingles, which typically strikes decades later and can cause severe long-term nerve pain or vision loss. Formally called herpes zoster, the disease infects about a third of people who have had chickenpox, usually showing up in older adulthood, according to the U.S. Centers for Disease Control and Prevention. Shingles infection rates have been increasing for more than two decades. Although the risk increases with age, children can develop it as well, especially if their immune system is weakened.

About 91 percent of U.S. children are vaccinated against chickenpox, according to the most recent National Immunization Survey data, but that does not necessarily mean they cannot get shingles. The chickenpox vaccine is made with the live attenuated (weakened) varicella virus, so “not surprisingly, it can also become latent after vaccination,” explains Anne A. Gershon, a professor of pediatric infectious disease at Columbia University. “The virus has been altered so the vaccine rarely causes symptoms, but once you’ve been immunized and after the natural infection, you carry the virus in your neurons for the rest of your life,” says Gershon, who wrote an editorial accompanying the new study, which was published in June in Pediatrics, and who was not involved in the work.

Previous research with small groups found conflicting results regarding shingles rates in children vaccinated against chickenpox, with lower rates in older children but higher rates in toddlers. In the new study, researchers analyzed the medical records of nearly 6.4 million children (ranging from newborns to 17-year-olds) who received care at six health care organizations in the West, Northwest and Midwest from 2003 to 2014. They looked at records from the child’s birth or entry into the health system up until age 18 (or leaving the system), so any shingles infections after age 18 were not included. Half the children were vaccinated for at least part of the full study period; the other half were not.

The authors found that one dose of vaccine reduced shingles infection by 78 percent—except in young toddlers. Shingles rates were significantly higher in vaccinated one-year-olds than unvaccinated ones, although this increased risk for vaccinated children vanished by age two. The authors suspect the higher risk in toddlers “could be related to the developing immune system in very young children,” says lead study author Sheila Weinmann, a senior investigator at the Center for Health Research, Kaiser Permanente Northwest in Portland, Oregon.

That does not mean delaying the vaccine past the recommended age of one year for the first dose is wise, she added. The longer children go without the first vaccine dose, the more likely they are to catch the wild chickenpox virus—“and maybe even pass it on to young infants who are too young to get vaccinated,” Weinmann says. “So it probably makes more sense to stick with the current recommendation.” (Three of Weinmann’s co-authors have received research funding for other studies from the pharmaceutical company Merck, which manufacturers the varicella vaccine.)

Even unvaccinated children appear to be benefiting from the vaccine’s use. Despite a brief shingles uptick in unvaccinated children from 2003 to 2007, overall rates in children declined by 72 percent from 2003 to 2014. Four years after the CDC began recommending the second varicella vaccine dose in 2006, shingles cases in unvaccinated children began dropping rapidly, likely because of herd immunity, Weinmann says. Herd immunity refers to the inability of a disease to travel easily through a highly vaccinated population. In this case, herd immunity’s effect on shingles rates would occur by protecting unvaccinated children from developing wild chickenpox in the first place, thereby preventing shingles later on. As they grow older, however, unvaccinated children would remain susceptible to chickenpox (and therefore shingles).

“This study makes it clearer than ever before that the benefits of the varicella vaccine go beyond simply preventing chickenpox,” says Nathan Boonstra, a general pediatrician at Blank Children’s Hospital in Des Moines, Iowa, who cohosts the podcast Vax Talk and was not involved in the study. “There’s very good evidence now that the vaccine prevents a serious complication of chickenpox down the road, and shingles is really awful,” especially since it can show up anywhere on the skin, including the face and eyes, he notes. This study’s large population size and 12-year duration, as well as the big difference in infection rates it found, will also help doctors explain the vaccine’s benefits to parents, Boonstra says.

Two vaccines exist against shingles: Zostavax for adults age 60 and older, and the much more effective Shingrix, approved in 2017, for adults age 50 and older. But it is not yet clear if children vaccinated against chickenpox will need a shingles vaccine in older adulthood. “We need to continue to follow a cohort of children who have been vaccinated and see what happens,” Gershon says, although she expects shingles will be less of a problem for them. There are not much data on adult shingles rates in the study group yet because the CDC first recommended the vaccine in 1996, so the first generation to receive it is currently in their early 20s. Shingles becomes much more common after age 50.

Nevertheless, the fewer children who are getting chickenpox in the first place, the fewer are likely to develop shingles later on. “Because vaccination coverage in the population has been increasing over time,” Weinmann says, “probably these [shingles] rates will continue to drop.”