WASHINGTON — The Food and Drug Administration is preparing to announce a new warning for the Johnson & Johnson coronavirus vaccine saying the shot has been linked to a serious but rare side effect – Guillain-Barré syndrome, in which the immune system attacks the nerves, according to four individuals familiar with the situation.

About 100 preliminary reports of Guillain-Barré have been detected after 12.8 million doses of Johnson & Johnson vaccine were administered, the Centers for Disease Control and Prevention said in a statement. These cases have largely been reported about two weeks after vaccination and mostly in men, many aged 50 years and older. Available data do not show a pattern suggesting a similar increased risk with the Pfizer-BioNTech and Moderna vaccines, after more than 321 million doses of those vaccines have been administered in the United States. The Guillain-Barré cases will be discussed as part of an upcoming meeting of CDC advisers, the agency said.

Guillain-Barré syndrome usually occurs at a rate of about 60 to 120 cases each week, according to CDC data. While the cause of the syndrome is not fully understood, it often follows infection with a virus, including influenza, or bacteria. Each year in the United States, an estimated 3,000 to 6,000 people develop the illness.

Most people fully recover, but some have permanent nerve damage, according to the CDC. People older than 50 are at greatest risk. In addition, about two-thirds of people who develop the syndrome experience symptoms several days or weeks after they have been sick with diarrhea or a lung or sinus illness.

Officials are expected to emphasize that the Johnson & Johnson vaccine is safe and that its benefits clearly outweigh the potential risks, according to the people familiar with the situation. And the association is far from definitive, said one of the individuals with knowledge of the situation: “It’s a real signal but a rare event.”

Johnson & Johnson and the FDA declined to comment.

Still, the warning will be the latest blow to a vaccine that had been widely anticipated for its ease of use – it requires only a single dose, which makes it especially helpful for harder-to-reach populations and regions. But the vaccine has been plagued by problems.

In April, federal officials paused use of the vaccine after it was linked to another rare side effect – severe blood clots. That pause was lifted within days after an extensive safety review by the FDA and CDC, and a warning was added to the Johnson & Johnson label.

The vaccine also has been hobbled by production problems at Johnson & Johnson’s subcontractor, Emergent BioSolutions, the only U.S. manufacturer of the vaccine. The Baltimore plant was shut down in April after federal officials discovered millions of doses had been contaminated with Astra Zeneca vaccine, which was also being made there.

Johnson & Johnson has had to throw away the equivalent of about 75 million doses of the vaccine. About another 40 million doses have been released for use. In response to the contamination, the Biden administration removed AstraZeneca manufacturing from the plant and put Johnson & Johnson in direct control of vaccine production there. But Emergent has not received authorization from the FDA to resume manufacturing the Johnson & Johnson product.

Other vaccines also have been associated with rare adverse events. The FDA in late June decided to add a warning to the Pfizer and Moderna coronavirus vaccines about mild, extremely unusual cases of myocarditis – heart inflammation – in some young adults and teens after vaccination. Federal health officials said there was “a likely association,” and that the problem appears most likely to occur in young men after they receive two doses of the vaccine.

The CDC and the Department of Health and Human Services, together with 15 of the country’s leading medical and public health organizations, issued a joint statement in June saying they “strongly encourage everyone 12 and older” to get the shots because the benefits far outweigh any potential harms.

In June, the American Neurological Association reported that two studies published in the journal Annals of Neurology had found 11 cases of Guillain-Barré syndrome two to three weeks after vaccination with the AstraZeneca vaccine. The cases, which were from England and India, involved an unusual variant of the disease that caused severe facial weakness, the organization said. An accompanying editorial described a similar case involving a Boston man who received the Johnson & Johnson vaccine.

Vaccine safety officials in Europe have recommended that a warning be added about Guillain-Barré to the AstraZeneca vaccine. But the committee said that while cases have been reported following vaccinations, “at this stage the available data neither confirms nor rules out a possible association with the vaccine.”

In 1976, there was a small increased risk of the syndrome after swine flu vaccination, which was a special flu shot for a potential pandemic strain of flu virus. A National Academy of Medicine review in 2003 found that people who received the 1976 swine flu vaccine had an increased risk, about one additional case of Guillain-Barré for every 100,000 people who got the swine flu vaccine. The exact reason for the link remains unknown.

The CDC monitors for Guillain-Barré syndrome during each flu season. The agency says the data on an association between seasonal influenza vaccine and the illness have been variable from season to season. When there has been an increased risk, it has consistently been in the range of one or two additional cases per 1 million flu vaccine doses administered, and that it is more likely that a person will get the illness after getting the flu than after vaccination.