U.S. regulators authorized another COVID-19 booster for people age 50 and older, a step to offer extra protection for the most vulnerable in case the coronavirus rebounds. The Food and Drug Administration’s decision Tuesday opens a fourth dose of the Pfizer or Moderna vaccines to those people at least four months after their previous booster. Until now, the FDA had cleared fourth doses only for people 12 and older who have severely weakened immune systems. The agency said this especially fragile group also can get an additional booster, a fifth shot. The latest expansion, regardless of people’…