Children who get vaccinated against chickenpox may have a lower risk of developing shingles, a painful rash caused by the chickenpox virus, U.S. researchers said on Friday.

A study of more than 170,000 children 12 and under who got Merck & Co. Inc's chickenpox vaccine between 2002 to 2008 found only 122 cases of shingles or 1 case in 3,700 children who got the vaccine, an unexpectedly low rate, the team reported in the Pediatric Infectious Disease Journal.

Shingles, sometimes called herpes zoster, is a painful recurrence of the chickenpox virus, which can lurk in the body for a lifetime. The infection usually starts with a rash on the face or body, and causes pain, itching or tingling.

"The message to parents and pediatricians is: vaccinating your child against the chickenpox is also a good way to reduce their chances of getting herpes zoster," said HungFu Tseng, a research scientist and epidemiologist at Kaiser Permanente in Pasadena, California.

"We decided to examine this because it is generally believed that the vaccine would reduce the risk of chickenpox, and the risk of childhood shingles is not well quantified," Tseng said in e-mailed comments.

Tseng said childhood shingles is not common. "We do know that it occurs more frequently in children with an immunodeficiency," Tseng said.

Using Kaiser Permanente's electronic health record database, the team identified all the children in Southern California who were vaccinated with chickenpox vaccine from 2002-2008. "We then went deeper into the electronic data to identify which cases had shingles diagnosis," Tseng said.

Among those children who had been vaccinated and developed shingles, one had lymphoid leukemia, one involved drug abuse, 16 had asthma, 12 had developmental disorders and 3 had psychological or mental disorders.

Prior studies of the incidence of shingles in children who have not been vaccinated for chickenpox have varied widely. Before the introduction of Merck's vaccine in 1995, the rate was 30 cases per 100,000 people per year.

Tseng said it is not clear if children who get the vaccine will have any protection as adults, even if they get a now-recommended booster shot.

"We don't know how long the protection will last, but with the new recommendation of two doses it is expected to be much longer," he said.