"We would call it a small increased risk," said W. Katherine Yih, a lecturer at Harvard Medical School who led one of the studies to examine the risks.
Rotavirus can cause severe gastroenteritis in infants, requiring hospitalization. The condition involves inflammation of the stomach and intestines, and its main symptoms are watery diarrhea and vomiting, according to the U.S. Centers for Disease Control and Prevention.
The benefits of the rotavirus vaccine -- preventing the viral infection and hospitalization of infants -- outweigh the risk of the bowel complication, Yih said. "In a typical five-year period following a typical year for births, the vaccine will probably prevent more than 50,000 hospitalizations."
Yih's team analyzed the bowel risk linked with one of the two rotavirus vaccines now on the market. She looked at RotaTeq, licensed in 2006. A team of researchers from the CDC and other institutions analyzed the risk linked with RotaTeq and another vaccine on the market, Rotarix, licensed in 2008.
Both studies are published online Jan. 14 in the New England Journal of Medicine.
A previous vaccine, RotaShield, was withdrawn from the U.S. market in 1999 due to the increased risk of the bowel problem, known as intussusception.
The RotaShield vaccine carried a risk of about one or two cases of bowel obstruction per 10,000 vaccine recipients, Yih noted. In her study, she looked at nearly 1.3 million doses of RotaTeq and found the vaccine is linked with about 1.5 cases of the bowel problem per 100,000 first doses. That is about one-tenth of the risk of RotaShield, she said.
As for Rotarix, there were not enough doses in her study to analyze risk, Yih added.
In the other study, researcher Eric Weintraub, of the CDC's Immunization Safety Office, and colleagues did not find an increased risk of bowel problems with RotaTeq. They did find that receiving two doses of the Rotarix vaccine was linked to 5.3 cases of bowel obstruction for every 100,000 infants vaccinated.
Both vaccines are given by mouth. RotaTeq requires three doses, given between 6 weeks and 32 weeks of age. Rotarix requires two doses, given between 6 weeks and 23 weeks.
The risks must be weighed against the benefits, Yih said. "The vaccine is very effective in preventing severe diarrhea," she said.
Dr. Roya Samuels, a pediatrician at Cohen Children's Medical Center, in New Hyde Park, N.Y., said that more research is needed to pinpoint exactly what the risk is.
These findings are not a reason to bypass or decline the vaccine, she said, also citing the benefits of preventing the viral infection.
"If I were the parent of an infant, I would not decline it," researcher Yih said of the vaccine. "I'd do it for the protection of my child, and those who can't receive it due to being immunocompromised." Among those with weakened immune systems, she said, are children on certain cancer treatments.
There's no way to know if some children are more prone to the bowel problem after getting the vaccine, Samuels said. Yih found the problem more likely after the first dose of RotaTeq.
Parents can be aware of possible symptoms, Samuels said. "If you notice your child in distress, pulling his knees up to the chest, with severe colic or blood in the stools -- any of those -- within seven days of administration of the rotavirus vaccine, [it] would warrant a trip to the emergency room," she said.
To learn more about rotavirus, see the U.S. Centers for Disease Control and Prevention.