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.
“If there are two cases, then this starts to look like a dangerous pattern,” said Mark Slifka, a vaccine expert at Oregon Health and Science University. “If a third case of neurological disease pops up in the vaccine group, then this vaccine may be done.”
A participant information sheet dated Sept. 11 on AstraZeneca’s trial in Britain lumped the two volunteers’ cases together, stating the illnesses were “unlikely to be associated with the vaccine or there was insufficient evidence to say for certain that the illnesses were or were not related to the vaccine,” based on safety reviews. The next day, AstraZeneca announced that it had resumed the trial in Britain.
But the F.D.A. has so far not allowed the company to start up again in the United States.
A spokesman for the F.D.A. declined to comment. The National Institutes of Health said in a statement that it “remains to be seen” whether the onset of illness in trial participants was coincidental or tied to the vaccine, adding that “pausing to allow for further evaluation is consistent with standard practice.”