The FDA said 100 preliminary reports of Guillain-Barré syndrome had been filed with the US government's Vaccine Adverse Reporting System, out of 12.8 million Janssen vaccines given.

"In most of these people, symptoms began within 42 days following receipt of the Janssen COVID-19 Vaccine. The chance of having this occur is very low," it adds.

People should seek medical attention if they notice symptoms such as weakness or tingling in the arms and legs, especially if it spreads, after receiving a vaccine, the FDA said.

Other telltale symptoms include difficulty walking, speaking, chewing or swallowing; double vision; and bowel or bladder control problems.

The FDA said 95 of the 100 reports of GBS involved people who needed hospitalization, and one person died.

"Each year in the United States, an estimated 3,000 to 6,000 people develop GBS. Most people fully recover from the disorder," the agency added.

"GBS has also been observed at an increased rate associated with certain vaccines, including certain seasonal influenza vaccines and a vaccine to prevent shingles.

Johnson & Johnson confirmed it was talking to the US Centers for Disease Control and Prevention and FDA about the issue.

"The chance of having this occur is very low, and the rate of reported cases exceeds the background rate by a small degree," it added.

So did Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases.

"You're always going to find some adverse event associated with vaccination," Fauci told CNN's Chris Cuomo.

"When you vaccinate tens of millions of people, you will always find a rare event. You've got make a decision -- does the benefit outweigh the unusual risk of an adverse event? Thus far with the vaccines, it's always been decided that the benefit of the vaccine outweighs the risk of an adverse event."

The CDC and FDA keep an eye out for Guillain-Barre syndrome.

"GBS is a neurological disorder in which the body's immune system damages nerve cells, causing muscle weakness or, in the most severe cases, paralysis. Each year in the United States, an estimated 3,000 to 6,000 people develop GBS; it is typically triggered by a respiratory or gastrointestinal infection. Most people fully recover from GBS," a CDC spokesperson told CNN.

"Reports of GBS after receipt of the J&J/Janssen COVID-19 Vaccine in the Vaccine Adverse Event Reporting System (VAERS) are rare, but do likely indicate a small possible risk of this side effect following this vaccine," the spokesperson added.

The spokesperson said the CDC's Advisory Committee on Immunization Practices would discuss the matter in an upcoming meeting.

"In the United States, nearly all COVID-19 hospitalizations and deaths are now occurring in unvaccinated people. The risk of severe adverse events after COVID-19 vaccination remains rare. Everyone age 12 years and older is recommended to receive a COVID-19 vaccine," the statement said.

It's yet another setback for the J&J vaccine, which has been bedeviled by problems.

J&J's Janssen vaccine was the third vaccine to win emergency use authorization from the FDA, after Pfizer's and Moderna's. According to the CDC, 84.6 million Americans have received both doses of Pfizer's vaccine; 62 million are fully vaccinated with Moderna's and 12.7 million with J&J's one-dose regimen.