According to an article in Stat, AstraZeneca (AZN 0.46%) has paused the U.S. clinical trial of its coronavirus vaccine, AZD1222, after a serious possible adverse event arose in a patient enrolled in a clinical trial testing AZD122 in the U.K.

At this point, the nature of the adverse event is not known, and it isn't clear whether it was caused by AZD122. The company is "working to expedite the review of the single event to minimize any potential impact on the trial timeline," according to Stat. The participant is expected to recover. 

In addition to the 30,000-participant U.S. study announced last week, AstraZeneca is also running clinical trials in the U.K., Brazil, and South Africa. The company plans to start studies in Japan and Russia as well. All told, the studies will enroll up to 50,000 participants globally.

Care provider giving a shot into an arm.

Image source: Getty Images.

It isn't publicly known exactly how many patients have received AZD1222, but investors can get some clues from Moderna (MRNA 1.58%), which had enrolled 21,411 participants in its late-stage coronavirus vaccine clinical trial as of Friday. Meanwhile, vaccine collaborators Pfizer (PFE 0.66%) and BioNTech (BNTX -2.07%) had enrolled 25,189 participants as of an update on Monday.

With that many people enrolled in coronavirus clinical trials, it's highly likely that one of the participants would develop a serious illness unrelated to the vaccination. Of course, proving that it's unrelated could be challenging and might only be possible by testing AZD122 on more participants. If the adverse event only occurs in one out of 50,000 participants, it's either unrelated or a side effect with such a low incidence level that it's a tolerable issue.

Shares of AstraZeneca were down 6.5% at 6:40 p.m. EDT in after-hours trading.