U.S. regulators 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 (FDA) said on May 5 that the shot should only be given to adults who cannot receive a different vaccine or specifically request J&J's vaccine. Under the new FDA instructions, J&J's vaccine could still be given to people who had a severe allergic reaction to one of the other vaccines and couldn't receive an additional dose. Still, regulators added warnings about the blood clots and a rare neurological reaction called Guillain-Barré syndrome. Less than 17 million Americans have received the J&J shot so far.