CHICAGO (CBS/AP) — A potential COVID-19 vaccine produced by AstraZeneca was one of two being tested in Chicago, but now, the company has put a temporary hold on its large-scale Phase III trial.

The company is investigating whether a patient suffered a serious side effect or an unrelated illness.

In a statement issued Tuesday, the company said its “standard review process triggered a pause to vaccination to allow review of safety data,” stating that a patient experienced a “potentially unexplained illness.” The health news site STAT first reported the pause in testing, stating that the impacted patient was in the United Kingdom.

An AstraZeneca spokesperson confirmed the pause in vaccinations covers studies in the U.S., which late last month began recruiting 30,000 people for the vaccine’s largest study.

It was not immediately clear what the impacts of the pause would be on that trial.

The vaccine, developed by Oxford University, is also being tested in thousands of people in Britain and in smaller studies in Brazil and South Africa.

Temporary holds of large medical studies are not unusual, and investigating any serious or unexpected reaction is a mandatory part of safety testing. AstraZeneca pointed out that it was possible the problem could be a coincidence; illnesses of all sorts could arise in studies of thousands of people.

“We are working to expedite the review of the single event to minimize any potential impact on the trial timeline,” the company statement said.

Dr. Ashish Jha of Brown University said via Twitter that the significance of the interruption was unclear but that he was “still optimistic” that an effective vaccine will be found in the coming months.

“But optimism isn’t evidence,” he wrote. “Let’s let science drive this process.”

The development came the same day that AstraZeneca and eight other drugmakers issued an unusual pledge, vowing to uphold the highest ethical and scientific standards in developing their vaccines.

Two other vaccines are in huge, final-stage tests in the United States, one made by Moderna Inc. that is also being tested in Chicago, and the other by Pfizer and Germany’s BioNTech. Those two vaccines work differently than AstraZeneca’s, and the studies already have recruited about two-thirds of the needed volunteers.

During the third and final stage of testing, researchers look for any signs of possible side effects that may have gone undetected in earlier patient research. Because of their large size, the studies are considered the most important phase of study for picking less common side effects and establishing safety.

The trials also assess effectiveness by tracking who gets sick and who doesn’t between patients getting the vaccine and those receiving a dummy shot.

The announcement follows worries that President Donald Trump will pressure the U.S. Food and Drug Administration to approve a vaccine before it was proven to be safe and effective.

The U.S. has invested billions of dollars in efforts to quickly develop multiple vaccines against COVID-19, but public fears that a vaccine is unsafe or ineffective could be disastrous — derailing the effort to vaccinate millions of Americans.

Representatives for the FDA did not immediately respond to requests for comment.

AstraZeneca’s U.S.-traded shares fell more than 6% in after-hours trading following reports of the trial being paused.

Northwestern University Feinberg School of Medicine put out a call in July for volunteers for a trial for the AstraZeneca vaccine.

Rush University Medical Center was also actively looking for volunteers for the AstraZeneca vaccine trial as recently as last week.

(© Copyright 2020 CBS Broadcasting Inc. All Rights Reserved. The Associated Press contributed to this report.)