The company has released few details about the two cases of serious illness in its trial. The first participant received one dose of the vaccine before developing inflammation of the spinal cord, known as transverse myelitis, according to a participant information sheet for AstraZeneca’s vaccine from July. The condition can cause weakness in the arms and legs, paralysis, pain and bowel and bladder problems.

The case prompted a pause in AstraZeneca’s vaccine trials to allow for a safety review by independent experts. A company spokeswoman told the Times last week that the volunteer was later determined to have a previously undiagnosed case of multiple sclerosis, unrelated to the vaccine, and that the trial resumed shortly thereafter…

The company said it had not confirmed a diagnosis in the second case, a participant who got sick after the second dose of the vaccine. A person familiar with the situation who spoke with The Times on the condition of anonymity said the participant’s illness had been pinpointed as transverse myelitis. The trial was paused again on Sept. 6 after she fell ill.

The condition is rare, but serious, and experts said that finding even one case among thousands of trial participants could be a red flag. Multiple confirmed cases, they said, could be enough to halt AstraZeneca’s vaccine bid entirely.