WASHINGTON (AP) — Federal health scientists say Pfizer's best-selling vaccine Prevnar is at least as effective as a competing vaccine from Merck at helping prevent potentially deadly infections in adultsPrevnar 13 is designed to reduce the risk of infection by pneumococcal disease, which causes ear infections, meningitis and pneumonia. The U.S. government recommends vaccination with Prevnar for all U.S. infants. But the latest figures from the Centers for Disease Control and Prevention indicate the disease also affects older adults, killing 5,000 of them each year.Pfizer Inc. has asked the FDA to expand Prevnar's approval for adults 50 and older.A review posted online by the FDA concludes that Prevnar generates an immune response at least as strong as Merck & Co.'s Pneumovax, the only pneumococcal vaccine currently approved for adults.