The nation's supply of vaccine for the impending flu (19) took a big hit Thursday when Chiron Corp. announced it had found tainted doses in its factory. The company said it will hold up shipment of about 50 million shots—about half the supply U.S. health (20) had hoped to have on hand this year—while it (21) what went wrong and determines whether the vaccine is safe to use. "There's no product (22) is going to go into the arms of the American public that will not have been (23) to have met the highest standards of (24) " chief executive Howard Pien said. Pien said the company hopes to ship between 46 million and 48 million doses by early October, about month later than (25) . About 1 million doses have already been shipped, but no vaccines have yet reached the (26) Pien said. Vaccinations usually begin in September and (27) through the flu season. Officials with the Centers for Disease Control and Prevention, which oversees the nation's flu vaccine (28) , did not immediately return calls for (29) . Chiron would not give (30) on the (31) of the contamination, which Pien said was found in a small number of (32) at the company's factory in Liverpool, England. The company supplies about half the nation's flu vaccine. Aventis Pasteur supplies most of the (33) .