The Food and Drug Administration yesterday notified pediatricians and parents of 28 cases of a potentially life-threatening condition in infants and children vaccinated against rotavirus, a leading cause of severe diarrhea worldwide.

The agency said the cases of intussusception, in which the intestine becomes twisted and blocked, were not necessarily caused by Merck & Co.'s year-old RotaTeq vaccine. The problem occurs naturally in about one in 2,000 youngsters.

The announcement, coming eight years after Wyeth was forced to withdraw its own rotavirus vaccine in response to similar reports of intestinal blockage, raises questions about a high-profile vaccine that was developed by Merck based on research done at Children's Hospital of Philadelphia and the nearby Wistar Institute.

"Of the 28 cases of intussusception, the number that many have been caused by the vaccine or occurred by coincidence is unknown," the FDA stated on its Web site. It said it issued the public health notification to encourage reporting of additional cases.

Paul A. Offit, chief of infectious disease at Children's Hospital and co-inventor of the vaccine, said the FDA's notice was contradictory and would unnecessarily scare doctors and parents.

"The fact that only 28 cases of intussusception have been reported after 3.5 million doses of the vaccine have been distributed is encouraging because it strongly suggests that no connection exists," Offit said.

Offit and Merck said the number of problems reported by the FDA was actually lower than what would be expected to occur in unvaccinated children.

In its public notice, the FDA also said the report "does not exceed the number expected" naturally. Noting that adverse events typically are underreported, the agency said its experience with the Wyeth rotavirus vaccine had led to data about problems "being evaluated continually."

It urged pediatricians and parents to report cases of intussusception online or by calling 1-800-822-7967.

Merck had conducted extensive clinical trials on the vaccine, including one involving 70,000 people that looked specifically for intussusception.

"We feel that physicians should continue to administer RotaTeq to appropriate infants," said Michelle Goveia, a Merck medical director. "There is no causal relationship with these cases."

Merck's stock rose 10 cents a share to $43.95 yesterday.

Both Wistar and Children's Hospital earn royalties from the sale of the vaccine that could be worth millions of dollars. In 2005 Wistar sold a portion of its anticipated royalties for $45 million days after an FDA advisory panel voted unanimously that RotaTeq was effective and did not appear to cause intestinal complications.

RotaTeq, given in three doses, was approved for sale in the U.S. early last February, and quickly recommended for inclusion among routine vaccinations.

Rotavirus infects millions of children worldwide, causing an estimated 500,000 deaths a year, mostly in developing countries. In the U.S., it is responsible for more than 400,000 doctor visits, 70,000 hospitalizations and as many as 60 deaths.

Contact staff writer Josh Goldstein at 215-854-4733 or