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WASHINGTON - The government warned on Tuesday of potentially life-threatening twisting of the intestines in infants vaccinated against a virus that is the leading cause of early childhood diarrhea.

The condition, called intussusception, is the same that led to the withdrawal of the first rotavirus vaccine eight years ago.

The Food and Drug Administration said it was unknown whether the recently approved vaccine, called RotaTeq, caused the 28 new cases. The condition also can occur spontaneously. Indeed, the reports don't exceed the numbers expected to occur naturally each year - the so-called background rate, the FDA said.

"It looks like this is the natural background rate that we are seeing," said Dr. Michelle Goveia, medical director for pediatric medical affairs at the vaccine's manufacturer, Merck & Co. Inc. Goveia suggested heightened concerns about the previous vaccine, made by Wyeth, prompted the FDA to act.

In Tuesday's public health notification, the agency said it wanted in part to encourage reporting of any additional cases of intestinal twisting or blockage to help it assess any risks associated with the three-shot vaccine series. It also said the vaccine's label would mention the cases of intussusception.

"It's a known serious, life-threatening adverse event that is being seen at an expected level postmarketing. But because it is so serious, we asked the company to change the label," FDA spokeswoman Karen Riley said.

Dr. Paul Offit, the vaccine's co-inventor, said the 28 reports were well below the hundreds of cases one would expect naturally. He suggested the FDA wanted to "shake the tree" for more reports about the vaccine.

"I am actually encouraged by those data: 28 cases, when you would have expected at least 500 cases, that is really reassuring," said Offit, of the Children's Hospital of Philadelphia. "I don't see how those numbers suggest something's awry. If anything, they suggest nothing's awry."

The 28 cases included 16 infants who required intestinal surgery. There have been no reports of deaths.

RotaTeq received FDA approval in February 2006. At the time, the FDA and Merck said trials of the vaccine involving nearly 70,000 infants indicated it did not increase the risk of intussusception. But Merck and the Centers for Disease Control and Prevention are conducting follow-up studies of tens of thousands more infants to track any long-term effects of the vaccine. The FDA also is monitoring reports.

About 3.5 million doses of the Merck vaccine have been distributed in the US, though not all have been used, the FDA said.

The earlier rotavirus vaccine, Wyeth's RotaShield, was pulled from the US market in 1999 after it was linked to a small increase in intussusception. It had been on the market a year.

In the United States, rotavirus sickens about 2.7 million children younger than 5, sends up to 70,000 to the hospital and causes 20 to 70 deaths each year.