WASHINGTON (AP) — U.S. health regulators have authorized an extra dose of the Pfizer or Moderna COVID-19 vaccines in people with weakened immune systems to better protect them from the virus. The announcement Thursday by the Food and Drug Administration applies to millions of Americans who take immune-suppressing medicines because of organ transplants, cancer or other disorders. The decision does not apply to otherwise healthy individuals. Health authorities are closely monitoring if and when the general population will need a booster shot but say for now, the vaccines continue to be highly effective in most healthy people.