A new vaccine that protects infants from rotavirus, which causes diarrhea and dehydration and leads to hundreds of thousands of deaths a year in developing countries, received federal approval Friday.

RotaTeq, made by Merck & Co., in trials prevented at least 98 percent of severe cases of gastroenteritis, or intestinal inflammation.

"This vaccine gives health care providers an important new tool that can effectively prevent an illness that affects almost all children within the first few years of life," said Dr. Jesse L. Goodman, director of the Food and Drug Administration's Center for Biologics Evaluation and Research.

In 1999, a different rotavirus vaccine, made by Wyeth, was pulled from the U.S. market after it was linked to a small increase in intussusception, a rare, life-threatening blockage or twisting of the intestine.

Trials of Rotateq, however, indicated it did not increase the risk of intussusception. But Merck officials have said they would conduct a follow-up study to track any long-term effects of their vaccine, which has been in development since 1993.

In the United States, the virus 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.

RotaTeq is a genetically engineered, three-dose oral vaccine that protects against five common rotavirus strains.

Among 5-year-olds in the U.S., an estimated one in 17 will visit the emergency room for rotavirus gastroenteritis, according to Merck. One out of 65 will be hospitalized.

A federal advisory committee on immunizations is to decide this month whether to add RotaTeq to the list of recommended pediatric vaccines, Merck said.

The liquid vaccine can be given orally to infants in three doses during regular well-baby visits to the doctor at two, four and six months.

Merck seeks to license the vaccine in more than 50 countries. It plans trials in Africa and Asia, where the virus is a noted killer.

"Virtually all children will experience rotavirus," said Dr. Paul Offit of The Children's Hospital of Philadelphia and co-creator of the vaccine. "While some children experience mild symptoms of rotavirus, infection can be severe and lead to dehydration that can be fatal."

Merck officials have said the vaccine is meant to be effective for two years, because an older child who gets rotavirus is much less vulnerable to dehydration and other severe consequences of the illness, and therefore less likely to need hospitalization. Children would still probably catch rotavirus and develop natural immunities to it.

Friday's approval should provide a boost for Merck, which faces thousands of lawsuits over its Vioxx painkiller. The vaccine will cost $62.50 a dose when ordered in 10-dose packets.

Side effects of the vaccine can include diarrhea, vomiting, ear infection, runny nose and sore throat, wheezing and coughing.

GlaxoSmithKline's Rotarix vaccine has been licensed in Mexico but has not yet come up for approval in the United States.