U.S. regulators on Monday added a new warning to Johnson & Johnson’s COVID-19 vaccine about links to a rare and potentially dangerous neurological reaction, but said it’s not entirely clear the shot caused the problem. The FDA announced the new warning, flagging reports of Guillain-Barre syndrome … Health officials described the side effect as a “small possible risk” for those getting the shot. The action comes after the FDA and the CDC reviewed reports of about 100 people developing the syndrome after receiving the one-dose vaccine. Almost all of were hospitalized and one person died, the FDA said.