A spokeswoman for the Food and Drug Administration declined to comment.
A person familiar with the situation, who spoke on condition of anonymity, said the participant who had the suspected side effect had been enrolled in a phase 2/3 study based in the UK. The person also said that a volunteer in the UK study was diagnosed with transverse myelitis, an inflammatory syndrome that affects the spinal cord and is often caused by viral infections. However, the timing of this diagnosis and whether it is directly related to AstraZeneca's vaccine is not yet known.
Transverse myelitis can result from a number of causes that trigger the body's inflammatory responses, including viral infections, said Dr. Gabriella Garcia, a neurologist at Yale New Haven Hospital. But, she added, the disease is often treatable with steroids.
AstraZeneca declined to comment on the participant's location and did not confirm the diagnosis of transverse myelitis. "The event is being investigated by an independent committee and it is too early to finalize the specific diagnosis," the company said.
Some said the company's halt was evidence that the process was working as it should.
"At this point in time, we don't know if the events that triggered the lockdown were vaccine-related," said Dr. Luciana Borio, who oversaw public health preparation for the National Security Council under Mr. Trump and served as a senior scientist at the FDA under President Barack Obama. "But it is important that they are thoroughly examined."
AstraZeneca's vaccine uses a viral vector that delivers coronavirus genes into human cells. The viral vector in this case is a modified chimpanzee adenovirus that has been modified so that it is harmless to humans. The coronavirus components of the vaccine are designed to trigger a protective immune response that is re-triggered when the actual coronavirus tries to infect a vaccinated person.
In an article 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 evaluate the safety of the product, had mild or moderate side effects, including Had muscle pain, and chills. However, none of the reactions were considered severe or life threatening and resolved quickly. The vaccine was considered safe enough to proceed with further testing.
AstraZeneca's vaccine is in phase 2/3 trials in England and India, and phase 3 trials in Brazil, South Africa and more than 60 locations in the US. The company intended to enroll 30,000 US members and began US lawsuits on August 31st.