Pascal Soriot, AstraZeneca’s CEO, said that if the trial is allowed to resume shortly, the company should still be ready to submit its vaccine to regulators for approval by year’s end, with widespread distribution beginning in the early part of 2021.

“It depends on the outcome of the review,” he said. “If the review allows us to restart, I think we are on track to have a set of data to submit [to regulators] before the end of the year,” he said.

AstraZeneca, which is developing a vaccine with researchers from the University of Oxford, is considered a front-runner in the race to deliver a COVID-19 inoculation that could help bring the global coronavirus pandemic to an end. The vaccine is currently undergoing late-stage human testing in the U.S., the U.K., Brazil, India, and South Africa.

But the pause in clinical trials may mean other companies are able to get their vaccine candidates approved first. Pfizer CEO Albert Bourla told Fortune on Wednesday that he remains optimistic there will be enough clinical trial data to know if the vaccine his company is developing is effective by the end of October.

Another vaccine, from the U.S. biotech company Moderna, is also undergoing large-scale clinical trials in the U.S., and may be ready for possible regulatory approval this year.

It is likely that several different vaccines will be necessary to immunize billions of people around the world.

Soriot, speaking at an event hosted by British publication Tortoise Media, said he had little insight into whether the trial would be cleared to resume, or how long the pause would last. Those decisions rest with an independent review board—made up of experts from different disciplines—that is investigating whether a serious medical condition experienced by one of the trial participants in the U.K. is likely linked to the vaccination.

“I do realize that people have lots of questions, and believe me I would like to have answers,” Soriot said. “But we have to be patient, and we have to do the right thing and follow the process.”

He said tests are being carried out to determine exactly why the person became ill. Several news organizations, citing people familiar with the case, reported that the person has a suspected case of transverse myelitis, an inflammation of the spinal cord that can be caused by an overactive immune response resulting from a vaccination.

Not the first time

Soriot said this diagnosis has not been confirmed. The company said Wednesday that the vaccine trials had also been paused once before, earlier in the summer, after one of the trial volunteers developed neurological symptoms. These were determined to be related to an undiagnosed case of multiple sclerosis unrelated to the vaccination, and the trials were allowed to resume, the company said.

The CEO said such testing pauses were the norm, not the exception, in clinical trials. He said this was especially true for vaccines, which are tested on healthy people, and therefore have a higher safety threshold for adverse side effects than medicines that are designed to treat people who are already ill.

Soriot said that if the trial is allowed to resume, there should be enough data soon to know if the vaccine is effective. “The answer will only come as the trials continue,” he said. “Does it protect people for a period of time? We should know that before the end of the year.”

He also said that even if AstraZeneca’s vaccine is approved this year, it would take time to distribute enough doses to vaccinate the entire world’s population. “Yes, I think we could still have a vaccine [by the start of 2021], but…we won’t have a lot of vaccine, of course, because you then have to scale up the manufacturing and distribution,” he said.

He said this was true despite AstraZeneca’s producing hundreds of millions of doses of the vaccine in parallel with the ongoing late-stage clinical trials.

Lives over profits

Various governments and international funding bodies have given AstraZeneca billions of dollars to fund this effort. The company has committed to not profiting from the vaccine until the coronavirus pandemic has been declared over.

Soriot bristled at suggestions AstraZeneca had not been transparent in not announcing the current clinical trial pause. The pause only became public because the health news site Stat broke the story on Tuesday. “When you conduct a clinical trial, you inform the regulators,” he said. “You don’t make big announcements in the press. This is a scientific process. You don’t hide anything, but you work with professionals—people with expertise.” He said as soon as the clinical trial was paused Oxford and AstraZeneca informed the appropriate government regulatory agencies in each country where trials were taking place.

He also defended rules that protect pharmaceutical companies from legal liability related to their development of a COVID-19 vaccine. Under U.S. law, companies working on a vaccine in response to a pandemic have immunity from lawsuits. In Europe, various governments have indemnified the companies against any legal costs.

Soriot said that in a pandemic, when billions of people are going to receive a vaccine, such rules were necessary to protect companies from being swamped with potentially frivolous lawsuits. “Otherwise no one would ever develop a vaccine,” he said.