Rotavirus gastroenteritis causes mild symptoms in many US patients, but severe cases lead to dehydration and 250,000 emergency-room visits a year. The illness is far more deadly in developing nations that lack facilities for rehydrating children intravenously.

The vaccine -- which is administered orally, by squirting just a few drops in the mouth -- was developed over an 11-year period by scientists at Children's Hospital of Philadelphia and the Wistar Institute.

Merck tested it on more than 70,000 infants in 11 countries, in one of the biggest clinical trials performed by a drug company. Merck, based in Whitehouse Station, NJ, has about 10,000 employees in the Philadelphia suburbs.

Standard Childhood Immunization?

The unusually large effort was prompted in part by a serious side effect associated with a previous rotavirus vaccine. That drug, made by Wyeth, was linked to blocked intestines in a small number of children and was withdrawn in 1999.

Testing of the Merck product has revealed no such pitfalls, company officials said.

A federal committee is meeting later this month to decide whether to recommend that the vaccine, dubbed RotaTeq, should be added to the standard US list of childhood immunizations. Another rotavirus vaccine, made by GlaxoSmithKline PLC, awaits FDA approval.

The Merck version is administered in three doses to infants. The drug is priced at $62.50 per dose when sold in a 10-dose pack, and requires no preparation by health practitioners.

"Literally, you just twist off the cap and administer," Merck spokeswoman Mary Elizabeth Blake said.

More Deadly in Developing Nations

Company officials declined to discuss sales projections. They said they were committed to making the drug available in developing countries at a lower price, by working with international health organizations.

First, however, the drug must be tested in some of those countries, where different types of the virus may be circulating. That process is beginning this year, Blake said.

The disease, called rotavirus gastroenteritis, causes mild symptoms in many US patients, but severe cases lead to dehydration and 250,000 emergency-room visits a year. The illness is far more deadly in developing nations that lack facilities for rehydrating children intravenously.

Live Virus 'Sort of Hybrid'

The vaccine was developed by Children's Hospital professors Fred Clark and Paul Offit and by Wistar professor emeritus Stanley Plotkin. It is based on a strain of rotavirus found in calves, which Clark first isolated in 1981.

The vaccine contains a live virus that is a sort of hybrid between the cattle rotavirus, which does not cause symptoms in humans, and versions of the microbe that are found in people.

It was made by co-infecting a cell culture with both types of viruses. The genetic material in the final product is 10 parts calf strain and 1 part human strain, said Plotkin, a renowned immunologist who is best known for developing the vaccine that eradicated rubella in the United States.

Merck shares closed up 57 cents at $34.39.

Contact staff writer Tom Avril at 215-854-2430 or tavril@phillynews.com.