Flu Vaccine Supply Cut in Half By LAURAN NEERGAARD     WASHINGTON (AP) -- Americans' supply of flu vaccine was abruptly cut in half Tuesday, prompting the government to ask most healthy adults to delay or skip flu shots so that the elderly and others most at risk from influenza can get scarce supplies.    British regulators unexpectedly shut down a major flu-shot supplier Tuesday, prompting a record shortage in this country just as flu season is about to begin...