Just a year after the federal government approved it, the world's only vaccine against the leading cause of childhood diarrhea was pulled off the market yesterday because of fears it may increase infants' risk of a dangerous bowel obstruction.

The government advised doctors in July to temporarily stop vaccinating babies against rotavirus after counting 20 infants who had developed bowel obstruction shortly after swallowing the vaccine. An estimated 1 million U.S. infants had been vaccinated.

The government has at least 99 reports of bowel obstruction possibly linked to the vaccine. So manufacturer American Home Products took the larger step of pulling its RotaShield vaccine off the market. Any doctors who still have doses should return them to the manufacturer immediately.

Parents should not worry if their babies took RotaShield without problems, said Jesse Goodman of the Food and Drug Administration. Cases of the bowel obstruction, called intussusception, thought linked to RotaShield occurred within the first week or two after vaccination.

This type of bowel obstruction, where a part of the intestine becomes enfolded within another, can occur by chance whether or not a baby ever had a rotavirus vaccine. One estimate suggests it strikes 50 of every 100,000 babies, with boys affected twice as often as girls. Symptoms include vomiting, bloody stools and abdominal pain. Surgery often is needed to clear the blockage if it isn't caught early.

Babies are most likely to contract intussusception between the ages of 5 months and 9 months, about the same age that babies received RotaShield.

A few bowel obstructions occurred during testing of RotaShield, so the FDA warned on the vaccine's label of a potential bowel obstruction side effect.

But as additional reports came in once RotaShield began selling, the question became whether RotaShield really did increase babies' risk, and if so, how much?

The government has studies under way to try to answer those questions but so far cannot say. Part of the investigation is to figure out how a vaccine could physically cause part of the intestine to slip out of place. One possible theory is that it might cause some bowel inflammation.

RotaShield was hailed last year as a long-awaited way to fight the leading cause of severe childhood diarrhea, a virus that attacks the lining of the small intestine. Sufferers, mostly under age 5, can experience 10 to 20 diarrhea episodes in a single day, quickly getting dangerously dehydrated.