The incident drew some concern among experts, who noted that a similar adverse neurological event, reported months ago, had occurred in another vaccinated volunteer. Although this earlier event prompted its own pause in AstraZeneca’s trials, an independent safety board ultimately determined it was unrelated to the vaccine, allowing studies to resume.
After the second AstraZeneca halt in September, trials abroad rapidly restarted in most countries. But the American hiatus persisted, with few details released as to why.
According to two vaccine experts familiar with the situation, the F.D.A. did not find the vaccine was responsible for the two neurological illnesses. But it could not rule out a link, either. The agency plans to alert study volunteers and ask specifically about related symptoms like weakness and numbness that might point to a milder case of transverse myelitis, the experts said.
Johnson & Johnson started its Phase 3 trial on 60,000 volunteers in September. On Oct. 12, the company announced its own trial pause, citing concerns that an illness had happened in one of its volunteers as well. The company has kept mostly quiet about the details of the incident.
“There are many possible factors that could have caused the event,” the company said. “Based on the information gathered to date and the input of independent experts, the company has found no evidence that the vaccine candidate caused the event.”
Adverse events are not uncommon in large-scale vaccine trials. In some cases, they are caused by a vaccine. But investigations usually reveal that they’re coincidental — a simple matter of chance.
Now that the trials are resuming, Dr. Borio said, changes may need to be made “to augment safety measures,” at least in the case of AstraZeneca, which, for instance, will likely need to monitor its volunteers for milder neurological symptoms, now that there’s precedent.