The fact sheet for Johnson & Johnson’s COVID-19 vaccine was revised to warn of the risk of a rare immune-system disorder, adding new headwinds for a shot once expected to be a linchpin of the U.S. immunization effort.

The Food and Drug Administration said in a statement Monday that it was adding the warning after 100 reports of Guillain-Barré Syndrome, a rare condition in which the immune system attacks the nerves, among people who had received the shot. So far, about 12.8 million Americans have been given the one-dose vaccine.

The agency said that 95 of the cases required hospitalization and that one person had died. While the available evidence suggests a link between the shot and the syndrome, the agency said that the data was insufficient to be able to say definitively it had caused the illness.

The revised fact sheet said symptoms began within 42 days of receiving the J&J shot.

According to the Centers for Disease Control and Prevention, the cases were seen mostly in men over the age of 50 about two weeks after being immunized. The CDC said that the issue will be discussed at an upcoming vaccine advisory committee meeting.

Between 3,000 and 6,000 people a year develop Guillain-Barré Syndrome, according to the FDA, and most fully recover from it. The agency said it has been seen at a higher rate with certain vaccines, including those for seasonal flu and shingles.

Cases generally start within days or weeks of a respiratory or gastrointestinal viral infection, and can result in mild spurts of muscle weakness or more severe forms of paralysis.

Despite the new warning, the CDC said the risk of severe adverse events from COVID vaccines remains remote, and that everyone 12 years and older is recommended to receive a vaccine. The FDA said the benefits of the J&J shot in preventing COVID-19 clearly outweigh known and potential risks.

COVID infections have been rising again in many areas due to the spread of the delta variant. Nearly all virus-related hospitalizations and deaths are now occurring among the unvaccinated, according to the CDC.


Another Setback

J&J said that it is in contact with the FDA and other health authorities about the Guillain-Barré cases.

“The chance of having this occur is very low, and the rate of reported cases exceeds the background rate by a small degree,” the company said in a statement.

The development marks another potential setback for J&J’s vaccine. The New Brunswick, New Jersey-based drugmaker’s shares fell 0.2% to USUS$169.48 at 4 p.m. in New York.

In April, U.S. health agencies urged providers to stop administering the shot to assess rare but serious blood clots that occurred among some people who had received it.

After a 10-day review, regulators lifted the pause, suggesting that the chance of developing the rare blood clot syndrome, thrombosis with thrombocytopenia, was very low, and the benefits of the shot outweighed the risks.

Regulators added information about the minimal risk of the clots to the shot’s fact sheet, noting most cases had occurred in women age 18 through 49.

Use of the J&J shot, which was once expected to be central to the U.S. vaccination campaign, has been hampered by concerns that it is less effective than other vaccines, as well as production problems at a company hired to manufacture them.

The Washington Post reported earlier that the U.S. Food and Drug Administration planned to add the warning about Guillain-Barré Syndrome.


GBS and Vaccines

Vaccines, by nature, have an effect on the immune system. Though they’re intended to bolster the body’s natural defenses, scientists have suggested it is biologically plausible that immunizations can be associated with Guillain-Barré Syndrome.

The CDC has long studied the association between the seasonal flu shot and 2009 H1N1 monovalent influenza vaccine and Guillain-Barré, though the agency has suggested the precise risk is difficult to estimate -- and that developing the syndrom from catching the flu itself is even more likely.

The FDA has made pharmaceutical companies include warnings about Guillain-Barré Syndrome with other vaccines. In March, for example, U.S. regulators required that GlaxoSmithKline Plc. revise the prescribing information for its shingles vaccine, Shingrix, having determined there was an association between it and Guillain-Barré.

In J&J’s large late-stage clinical trial, one vaccine recipient and one placebo recipient developed Guillain-Barré Syndrome within two weeks of receiving an injection, falling short of establishing a causal association to the vaccine.


Other Warnings

Health officials have warned about possible rare side effects with other COVID-19 vaccines. Last month, the FDA added warnings to the patient and provider fact sheets for messenger RNA shots made by Pfizer Inc.-BioNTech SE and Moderna Inc. about a small risk of possible heart inflammation.

Roughly 1,200 cases of myocarditis and pericarditis have been reported in people who had received the shots, according to the CDC, while about 147 million Americans have been fully vaccinated with them.

The available data don’t show a pattern of Guillain-Barré Syndrome in recipients of mRNA vaccines, the FDA and CDC said.

--With assistance from Drew Armstrong.