Last Sunday, AstraZeneca suspended its trial after a participant in Britain became seriously ill. The company did not announce the decision. On Wednesday, after the news organization Stat reported that the trial had paused, AstraZeneca released a statement that described it only as a “potentially unexplained illness.”

The New York Times reported that a person familiar with the situation, who spoke on the condition of anonymity, had said that the symptoms were consistent with inflammation of the spinal cord, known as transverse myelitis. The condition can be treated and is typically resolved in a few months, but severe attacks can cause major disabilities.

“Good news for everyone: The Oxford vaccine trials are back up and running,” he said. “This pause shows we will always put safety first. We will back our scientists to deliver an effective vaccine as soon as safely possible.”

Michele Meixell, a spokeswoman for AstraZeneca, said that the company could not disclose any further medical information about the sick volunteer.

Ms. Meixell did not say when she expected the trials in other countries to restart. “AstraZeneca will continue to work with health authorities across the world and be guided as to when other clinical trials can resume,” she said.

Amy Rose, a spokeswoman for Pfizer, said that the additional volunteers might be recruited in any of the countries where trials are taking place, including Argentina and Brazil. “We will continue to actively recruit all eligible individuals where the incidence and spread of COVID-19 cases is greatest,” she said in an email.

She also said that a bigger trial might allow Pfizer to get those results sooner.

“Enrolling more participants gives us greater flexibility and allows us to potentially reach the number of cases needed to complete the study faster,” she said.

Dr. Topol said he found it unlikely that Pfizer could get full results from a 44,000-person trial by the end of October, given that the vaccine requires two doses separated by three weeks. It was possible, he said, that Pfizer intended to look at the early results of the trial to see how effective the vaccine was.

An outside committee of experts can evaluate clinical trials at different points in the study. The panel could advise the company to stop the trial if it showed overwhelming efficacy, if it failed to prevent cases of Covid-19 or if there were serious safety concerns. Pfizer has not said when those so-called interim analyses would take place or what the criteria are for stopping the trials.

“We are not going to speak to timing or specifics of any interim analyses,” said Ms. Rose, the Pfizer spokeswoman.

Dr. Topol warned that because Pfizer has reiterated its goal of getting results by October even as it is recruiting new participants, “What we see here from this Pfizer news release is that they fully intend to interrupt the trial.”

On Tuesday, Pfizer and eight other companies that are developing coronavirus vaccines signed a pledge that they would wait to put forward a vaccine until the evidence for safety and efficacy was clear.

“We are committed to running our trial with scientific excellence and integrity,” Ms. Rose said.

This content was originally published here.