The US Food and Drug Administration (FDA)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, an immune system disorder that can cause muscle weakness and occasionally paralysis. Health officials described the side effect as a “small possible risk” for those getting the shot.

The syndrome has occurred in some people who have received the Johnson & Johnson vaccine, the FDA in a letter on Monday. In most of these people, symptoms began within 42 days following injections of the vaccine, it added. The chance of having this occur is very low, the agency said.

About 100 preliminary reports of Guillain-Barre syndrome have been detected in vaccine recipients after the administration of 12.8 million doses of the Johnson & Johnson jab in the US, The Washington Post quoted the Centers for Disease Control and Prevention (CDC) as saying in a statement. Of these reports, 95 were serious and required hospitalization, and one had dead.

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