Moncef Slaoui, the head of Operation Warp Speed, said he expects the U.S. trials of vaccines made by AstraZeneca and Johnson & Johnson to restart as soon as this week.

The two companies developing COVID-19 vaccines backed by Operation Warp Speed temporarily halted their trials because participants fell ill, slowing the race for a shot to halt the pandemic. J&J paused its trial last week when a participant got sick. AstraZeneca’s trial paused last month after a woman in the U.K. study developed neurological symptoms and the U.S. Food & Drug Administration has yet to clear the study to resume in the U.S.

"It's for the FDA to announce and decide, but I understand that this is imminent," Slaoui said in an interview, when asked if AstraZeneca could resume its trial this week. "I hope that the J&J trial also will restart later this week."

Both vaccines use a viral vector based on adenoviruses to stimulate an immune response. "I have not seen data at all that suggests these platform technologies have a problem," he said in an interview.

Johnson & Johnson is testing its vaccine in as many as 60,000 volunteers from Peru to South Africa. U.S. regulators have not put a clinical hold on J&J's trial. British drugmaker Astra and partner the University of Oxford last month temporarily stopped tests of their vaccine candidate after a trial participant fell ill. While the Astra study has remained halted in the U.S., where it is being evaluated by regulators, it has resumed in a number of other countries.

Slaoui said there have been two confirmed cases of transverse myelitis, or inflammation of the spinal cord, in the AstraZeneca trials, one in the vaccine group and one in the placebo group. The company has not confirmed the adverse events were transverse myelitis. He said regulators need to determine whether adverse events are caused by the vaccine or are just a coincidence.

"If it turns out you can't demonstrate that it was associated, you restart the trial," he said. "But you continue to pay attention very, very carefully to that kind of family of side effects. And in this case it would be central nervous system side effects."

Pauses are common in large-scale trials involving thousands of participants. The compression of COVID-19 vaccine development timelines, which typically take years, has heightened concerns around vaccine safety.

One participant died during the Astra vaccine trial in Brazil, sending the company's shares down. But a person familiar with the matter said the deceased person hadn't taken the company's vaccine and was in the control group. Astra's trials have resumed in the U.K., Brazil, South Africa, India and Japan.

Japan's Ministry of Health has requested details from AstraZeneca on the death in the Brazil trial. The ministry continues to work with companies to ensure proper vaccine trials are taking place in Japan, Chief Cabinet Secretary Katsunobu Kato said Thursday.

Other types of vaccines employing even newer, RNA-based technologies are being tested as inoculations against COVID-19. The U.S. government, through Operation Warp Speed, has been funding the rapid development of vaccines to stem the tide of this deadly pandemic.