WASHINGTON, D.C.: US regulators on Monday (Tuesday in Manila) 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 Food and Drug Administration (FDA) announced the new warning, flagging reports of Guillain-Barré syndrome, an immune system disorder that can causes muscle weakness and occasionally paralysis. Health officials described the side effect as a 'small possible risk' for those getting the shot.