WASHINGTON (Reuters) - GlaxoSmithKline Plc (GSK.L: Quote, Profile, Research) won U.S. approval on Thursday to sell an oral vaccine to prevent a leading cause of severe diarrhea in infants.

The Food and Drug Administration cleared the Rotarix vaccine for preventing severe diarrhea from the rotavirus, which causes about 55,000 hospitalizations in U.S. children each year. The virus also kills more than 600,000 children worldwide, mostly in developing countries.

Rotarix already is approved in more than 100 other countries throughout the world. The vaccine competes with Merck & Co Inc's (MRK.N: Quote, Profile, Research) rotavirus vaccine called RotaTeq.

Sales of Rotarix were $182 million in 2007 and are expected to rise with approval in the large U.S. market. The vaccine will be commercially available in the United States in the second half of the year, Glaxo said.

"Rotarix will offer protection against the most commonly circulating rotavirus types in the U.S. and allow infants to complete the vaccination series by four months of age," the company said in a statement.

(Reporting by Lisa Richwine; Editing by Tim Dobbyn)