The approval is welcome news for GlaxoSmithKline, which announced worldwide layoffs last year after its second-leading seller, the diabetes drug Avandia, was linked to heart attacks.

But the firm's new vaccine faces stiff competition from Merck & Co. Inc.'s Rotateq, which won approval two years ago and rang up $525 million in sales in 2007.

Leonard Friedland, a GlaxoSmithKline senior director in pediatric vaccines research, said his firm's product had an advantage because it could be given in two doses, rather than three for Merck's Rotateq. The smaller number of doses is a convenience for patients and caregivers.

Merck executives counter that their vaccine is easier to administer. A caregiver has to twist off the lid and squeeze it into a baby's mouth, while GSK's vaccine will have to be mixed in the office and prepared in advance.

Friedland acknowledged that GlaxoSmithKline was working on a similar liquid delivery, but could not say when it would be available.

Both vaccines appear to be highly effective, though the firms disagree about which product has wider coverage of virus types.

Merck's earlier foray in the market could help it maintain its market share. Another rotavirus vaccine, Wyeth's RotaShield, had to be withdrawn in 1999 because it was linked to a rare condition - intussusception - in which the bowel enfolds on itself, blocking the system.

Merck already has distributed 10 million doses of its Rotateq without reprising Wyeth's past problems, executives have said. Any new vaccine will be carefully watched.

GlaxoSmithKline's Friedland said his company's product already was available in 100 countries and was also safe.

Both firms, each with major operations in the Philadelphia region, could compete over cost. Merck's vaccine has a catalog price of $66.83 for a private-sector dose and $55 for a public-agency dose. GlaxoSmithKline would not disclose its price yesterday.

Merck, which is based in New Jersey, has its vaccine division headquarters in West Point, Pa. GlaxoSmithKline, which is based in London, has a U.S. headquarters in Philadelphia.

Diarrhea caused by rotavirus causes more than half a million deaths worldwide every year.

Before the vaccine, rotavirus infections caused 60 deaths a year in the United States, along with 3 million cases of diarrhea and up to 70,000 hospitalizations for diarrhea and dehydration in children younger than 5.

Frequent hand-washing is the best method to limit the spread of rotavirus infection.