The FDA has approved Merck’s RotaTeq (rotavirus vaccine, live, oral, pentavalent) to prevent rotavirus gastroenteritis, a leading cause of severe diarrhea in infants and young children that kills nearly 500,000 children globally each year.Analysts forecast the vaccine could bring Merck as much as $500 million annually by 2009. RotaTeq is the second vaccine for rotavirus to be put on the market. The first, Wyeth’s RotaSheild, was withdrawn just 10 months after it debuted in September 1998.  The drug maker could start fielding orders from doctors immediately and would be able to ship doses as early as this week.