U.S. regulators added a new warning to Johnson & Johnson’s COVID-19 vaccine about links to a rare and potentially dangerous neurological reaction.

The announcement Monday said it's not entirely clear the shot caused the problem. 

Vaccine regulators found the risk to be low, but say Johnson & Johnson’s vaccine recipients are appearing to be three to five times more likely to develop an autoimmune disorder known as Guillain-Barre.

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The government said there have been reports of 100 people who got the shot developing an immune system disorder that can causes muscle weakness and occasionally paralysis. Most of them were serious cases requiring hospitalization

Dr. Fauci shared his insights on the new information saying in part, “The risk of the disease, when you balance the risk of the disease versus the risk of this very rare adverse event, overwhelmingly favors the fact that you should get this vaccine. You're always going to find some adverse event associated with vaccination.”

The reports represent a tiny fraction of the nearly 13 million Americans who have received the one-dose vaccine. According to the FDA, 10 out of every million people in the U.S. develop the condition every year and most recover.

The announcement said the vaccines most used in the U.S., made by Pfizer and Moderna, show no risk of the disorder.

AP wire services helped contribute to this report.