The Food and Drug Administration is adding a warning label to the Johnson & Johnson Covid-19 vaccine following reports of 100 cases of Guillain-Barré syndrome in vaccine recipients.
Ninety-five of those cases were serious enough to require hospitalization, and one person died, the FDA said in a statement Monday. Guillain-Barré syndrome damages nerve cells and can cause muscle weakness. In rare cases, it can cause paralysis.
The cases, which are now under investigation by the FDA and the Centers for Disease Control and Prevention, "have largely been reported about two weeks after vaccination and mostly in males, many aged 50 years and older," the CDC said in a statement.
The warning was added to provide information on the "observed increased risk" of the syndrome, the FDA said, but added that more research is needed to determine whether vaccination was the definitive cause. The agency noted that the benefits of Covid-19 vaccination clearly outweigh the risks.
The warning only applies to the Johnson & Johnson vaccine, not to the Covid-19 vaccines from Pfizer-BioNTech or Moderna, both of which use a different kind of technology.
Johnson & Johnson said in a statement that the company had been in contact with the FDA "and other regulators about rare cases of the neurological disorder," adding that the number of reports are only slightly higher than the number that would normally be expected, regardless of whether an individual had been vaccinated.
"The chance of having this occur is very low," the company said.
If confirmed, the 100 cases would represent a tiny fraction — less than 0.001 percent — of the more than 12.8 million Johnson & Johnson doses that have been administered in the United States.
The CDC statement said one of its advisory committees will discuss the possible link in an upcoming meeting.
Download the NBC News app for full coverage of the coronavirus outbreak
In April, administration of the Johnson & Johnson shots was paused to investigate very rare blood clots following the vaccine.
In June, the FDA told the company to discard 60 million doses of vaccine that had been contaminated with ingredients from another vaccine at the Emergent BioSolutions plant in Baltimore.
Guillain-Barré syndrome is rare, affecting an estimated 3,000 to 6,000 people each year in the U.S, typically following a viral or bacterial infection, according to the CDC. It has also been linked to other vaccines, including certain seasonal flu vaccines and a vaccine to prevent shingles.
Most people recover fully; however, permanent nerve damage and paralysis are possible. People over age 50 are most at risk.
The FDA's warning label will be included in the fact sheets provided to vaccine recipients.
Those who have received the Johnson & Johnson vaccine should contact a doctor if they develop weakness or tingling sensations, especially in the arms or legs, as well as any sudden problems walking, speaking, chewing or swallowing, the FDA said.