Infants now have another vaccine to swallow: The Food and Drug Administration has approved the first vaccine against the leading cause of childhood diarrhea.

Doctors say the vaccine, called Rota-Shield, should prevent thousands of American preschoolers from being hospitalized with severe diarrhea. But the vaccine's real promise lies in developing countries, where the intestinal infection called rotavirus kills up to 1 million children every year."The thing that will probably limit its use in developing countries, unfortunately, may be cost," said Dr. Karen Goldenthal, the FDA's vaccine director. "It's got to be incredibly cheap to be implemented."

Rotavirus is the leading cause of childhood diarrhea, attacking the lining of the small intestine. In severe cases, a child can experience 10 to 20 episodes of diarrhea in a single day, quickly getting dangerously dehydrated. About 75 percent of all children are infected before age 5; the youngest get the sickest.

Good health-care limits rotavirus-caused deaths in this country to no more than 40 a year. But 3 million American children are sickened by rotavirus annually, and 55,000 are hospitalized. The medical bills total $400 million.

The drug, discovered by the National Institutes of Health but manufactured by Wyeth-Ayerst Laboratories, won't wipe out rotavirus but it is expected to make a big dent in those numbers. Studies show the drug can cut rotavirus illness in half.