U.S. regulators on Thursday strictly limited who can receive Johnson & Johnson’s Covid-19 vaccine due to a rare but serious risk of blood clots. The Food and Drug Administration said the shot should only be given to adults who cannot receive a different vaccine or specifically request J&J’s vaccine. U.S. authorities for months have recommended that Americans starting their Covid-19 vaccinations use the Pfizer or Moderna shots instead. FDA officials said in a statement that they decided to restrict J&J’s vaccine after taking another look at data on the risk of life-threatening blood clots within two week of vaccination.