Johnson & Johnson’s beleaguered Covid-19 vaccine may be associated with a small increased risk of Guillain–Barré syndrome, a rare but potentially serious neurological condition, federal officials said on Monday. The Food and Drug Administration has added a warning about the potential side effect to its fact sheets about the vaccine.

The risk appears to be very small. So far, there have been 100 reports of the syndrome in people who had received the Johnson & Johnson vaccine. Nearly 13 million doses of the vaccine have been administered in the United States.

Here are answers to some common questions about the syndrome and its connection to vaccination.

What is Guillain-Barré syndrome?

Guillain-Barré is a rare condition in which the body’s immune system attacks nerve cells. It can cause muscle weakness and paralysis. Although the symptoms often pass within weeks, in some cases, the condition can cause permanent nerve damage. In the United States, there are typically 3,000 to 6,000 cases of the syndrome per year, according to the Centers for Disease Control and Prevention. It is most common in adults over 50.

The precise cause of the syndrome is unknown, but in many cases the condition follows another illness or infection, such as the flu. It has also been reported in people with Covid-19.

What does it have to do with vaccination?

This is not the first vaccine that has been linked to Guillain-Barré, although the risk appears to be tiny. A large swine flu vaccination campaign in 1976 led to a small uptick in the incidence of syndrome; the vaccine caused roughly one extra case of Guillain-Barré for every 100,000 people vaccinated. The seasonal flu shot is associated with roughly one to two additional cases for every million vaccines administered.

“I think the data are pretty compelling that the flu vaccine causes Guillain-Barré syndrome, but it’s a very small risk,” said Daniel Salmon, the director of the Institute for Vaccine Safety at Johns Hopkins University.

The shingles vaccine Shingrix may also increase the risk of the condition.

It is not entirely clear why some vaccines may cause Guillain-Barré. “We don’t really understand the biological mechanism,” Dr. Salmon said. “It’s an incredible frustration.”

What do we know about its connection to the Covid-19 vaccines?

One hundred reports of the syndrome after vaccination with the Johnson & Johnson shot have been submitted to the federal Vaccine Adverse Event Reporting System (VAERS), officials said on Monday. Of those, 95 cases resulted in hospitalization, and one was fatal.

The syndrome was generally reported about two weeks after vaccination, primarily in men, many of whom were 50 or older, officials said. There is not yet enough evidence to establish that the vaccine causes the condition, but the F.D.A. will continue to monitor the situation, the agency noted in a statement.

There is not yet any data to suggest a link between the condition and Covid-19 vaccines made by Pfizer-BioNTech or by Moderna, both of which rely upon a different technology, the F.D.A. said.

What signs and symptoms should I look out for?

The syndrome is most likely to appear within 42 days of vaccination, the F.D.A. notes in its revised fact sheet for patients. You should consult with a doctor if you begin to experience weakness or tingling in your arms and legs, double vision or difficulty walking, speaking, chewing, swallowing or controlling your bladder or bowels.

Should I still get a Covid-19 vaccine?

If the link between the vaccine and Guillain-Barré is real, it appears to be far outweighed by the risks of Covid-19, experts said. In the United States, almost all hospitalizations and deaths from Covid-19 are happening in those who are unvaccinated, the C.D.C. said in a statement. The agency recommends that everyone who is 12 or older be vaccinated.

“Everything has risks,” Dr. Salmon said. “And the key to decision-making is to optimize the benefits and reduce the risks.” He added, “Covid is a pretty nasty disease that’s killed 600,000 people.”