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The US Food and Drug Administration on Monday added a warning to the fact sheet for Johnson & Johnson's Covid-19 vaccine, saying data suggests an increased risk of a rare neurological disorder in the six weeks after inoculation.
In a letter to the company, the FDA classified the chances of getting Guillain-Barre syndrome after vaccination was "very low".
But it said Johnson & Johnson vaccine recipients should seek medical attention if they have symptoms including weakness or tingling sensations, difficulty walking or difficulty with facial movements.
About 12.8 million people have received Johnson & Jonson's one-dose vaccine in the US.
The FDA said 100 preliminary reports of the syndrome in the vaccine recipients include 95 serious cases that required hospital treatment and one reported death.
Johnson & Johnson said it was in discussion with regulators about the cases of Guillain-Barre.
It said the rate of reported cases among Johnson & Johnson vaccine recipients exceeded the normal rate by only a small degree.
Guillain-Barre syndrome is a rare neurological condition in which the body's immune system attacks the protective coating on nerve fibres.
Most cases follow a bacterial or viral infection and most people fully recover from it.
The condition has been linked in the past to vaccinations, most notably for a swine flu outbreak in the US in 1976, and decades later to the vaccine used during the 2009 H1N1 pandemic.
The US Centres for Disease Control and Prevention said most of the cases were in men, many of whom were 50 or older.
It did not find higher than expected cases of Guillain-Barre in recipients of the mRNA-based vaccines from Pfizer-BioNTech and Moderna.
Last week, European regulators recommended a similar warning for AstraZeneca's Covid-19 shot, which is based on similar technology to that of Johnson & Johnson's vaccine.
The warning is another setback for the Johnson & Johnson shot, which was supposed to be an important tool for vaccinating people in hard-to-reach areas and among those hesitant to be vaccinated.
It requires only one shot and has less stringent storage requirements than the Pfizer or Moderna vaccines.
But use of the vaccine has already been linked to a very rare, potentially life threatening blood-clotting condition and slowed by production problems at the main plant where it is being made.
US regulators decided in April that the vaccine's benefits outweighed the risk of blood-clotting.
The warning was first reported by The Washington Post on Monday.
