Late-stage coronavirus vaccine trials run by AstraZeneca and Johnson & Johnson have resumed in the United States after the companies said Friday that serious illnesses in a few volunteers appeared not to be related to the vaccines.
Federal health regulators gave AstraZeneca the green light after a six-week pause, concluding there was no evidence the experimental vaccine had directly caused neurological side effects reported in two participants. The AstraZeneca news was first reported by The Wall Street Journal.
Johnson & Johnson said that its trial, which had been on pause for 11 days, would restart after a company investigation determined that a “serious medical event” in one study volunteer had “no clear cause.” To maintain the integrity of the trial, the company said, it did not check whether the volunteer received the vaccine or the placebo.
Dr. Luciana Borio, a former acting chief scientist at the Food and Drug Administration, welcomed the announcements, citing the urgent need for multiple vaccines to remain in the race for a product that could protect the global population from the coronavirus, which has already killed more than a million people worldwide.
“The demand for safe and effective Covid vaccines exceeds any single manufacturer’s production capacity,” Dr. Borio said. “We really need several in the field.”
An F.D.A. spokesperson declined to comment on Friday afternoon.
AstraZeneca and Johnson & Johnson are two of the four companies now in late-stage clinical trials in the U.S. for experimental coronavirus vaccines. Both companies are using adenoviruses, which typically cause harmless colds. The adenovirus is engineered so that it can chauffeur a coronavirus gene into human cells.
Their two high-profile competitors, Moderna and Pfizer, also in advanced trials, are instead using a technology based on genetic material known as mRNA. Delivered into human cells, the mRNA prompts the production of coronavirus proteins, triggering an immune response.
AstraZeneca moved swiftly into clinical trials, enrolling thousands of volunteers for its vaccine trials around the world in countries including Brazil, India, South Africa and Britain. A large, late-stage trial kicked off in the United States at the end of August. But all the trials were halted days later, on Sept. 6. A volunteer who had received the vaccine in the United Kingdom reportedly experienced symptoms of transverse myelitis, or inflammation of the spinal cord, triggering a global pause to the company’s efforts.
The incident sparked some concern among experts, who noted that a similar adverse neurological event, reported months ago, had occurred in another vaccinated volunteer. While 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.
Following the second AstraZeneca halt in September, trials abroad rapidly resumed in most countries. But the American hiatus persisted, with few details released as to why.
According to two vaccine experts familiar with the situation who were not authorized to discuss it publicly, the F.D.A. did not directly tie the vaccine to the two neurological illnesses, although it could