A spokeswoman for the Food and Drug Administration declined to comment.
A person familiar with the situation, who spoke on the condition of anonymity, said that the participant who experienced the suspected adverse reaction had been enrolled in a Phase 2/3 trial based in the United Kingdom. The individual also said that a volunteer in the U.K. trial had received a diagnosis of transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often sparked by viral infections. However, the timing of this diagnosis, and whether it was directly linked to AstraZeneca’s vaccine, is still unknown.
Transverse myelitis can result from a number of causes that set off the body’s inflammatory responses, including viral infections, said Dr. Gabriella Garcia, a neurologist at Yale New Haven Hospital. But, she added, the condition is often treatable with steroids.
AstraZeneca declined to comment on the location of the participant and did not confirm the diagnosis of transverse myelitis. “The event is being investigated by an independent committee, and it is too early to conclude the specific diagnosis,” the company said.
Some said the company’s halt was evidence that the process was working as it should.
“At this stage, we don’t know if the events that triggered the hold are related to vaccination,” said Dr. Luciana Borio, who oversaw public health preparedness for the National Security Council under Mr. Trump and who was acting chief scientist at the F.D.A. under President Barack Obama. “But it is important for them to be thoroughly investigated.”
AstraZeneca’s vaccine uses a viral vector that ferries coronavirus genes into human cells. The viral vector in this case is a modified chimpanzee adenovirus, altered to render it harmless to people. The coronavirus components of the vaccine are intended to spark a protective immune response that would be roused again should the actual coronavirus try to infect a vaccinated individual.
In a paper published in The Lancet in July, researchers behind AstraZeneca’s formulation reported that the majority of participants in the vaccine’s Phase 1/2 trials, which are designed to assess the product’s safety, had experienced some mild or moderate side effects, including muscle aches and chills. None of the reactions, however, were considered severe or life-threatening, and resolved quickly. The vaccine was deemed safe enough to proceed to further testing.
AstraZeneca’s vaccine is in Phase 2/3 trials in England and India, and in Phase 3 trials in Brazil, South Africa and more than 60 sites in the United States. The company intended for its U.S. enrollment to reach 30,000, and started its American trials on Aug. 31.