Federal health officials are looking into "around 100 preliminary reports" of a rare but potentially serious neurological problem in people who have received Johnson & Johnson's COVID-19 vaccination.
In a statement, the Centers for Disease Control and Prevention said the agency and the Food and Drug Administration are monitoring the reports of Guillain-Barré syndrome, which damages nerve cells and can cause muscle weakness.
The cases are only linked 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.
If confirmed, the 100 cases would represent a tiny fraction — less than 0.001% — 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.
Any warning label, if warranted, would come from the FDA. The FDA does not need to wait for the CDC advisory meeting to make a decision.
"COVID-19 vaccine safety is a top priority for the federal government, and we take all reports of health problems following COVID-19 vaccination very seriously," the CDC statement read.
The CDC said the cases under investigation "have largely been reported about two weeks after vaccination and mostly in males, many aged 50 years and older."
Johnson & Johnson and the FDA did not immediately respond to requests for comment.
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.
Most people recover fully; however, permanent nerve damage and paralysis are possible. People over age 50 are most at risk.
This story was originally published on NBC News.