AstraZeneca has paused a late-stage trial of its promising COVID-19 vaccine candidate while it evaluates a single UK patient who may have experienced a serious adverse reaction.

The drugmaker said in a statement that its “standard review process was triggered, and we voluntarily paused vaccination” in the U.S. phase 3 trial in order to assess the safety data. AZ did not elaborate on the nature of the possible adverse reaction except to describe it as “a potentially unexplained illness,” meaning it’s possible it may not have been caused by the vaccine.

“We are working to expedite the review of the single event to minimize any potential impact on the trial timeline,” the company said in its statement. “We are committed to the safety of our participants and the highest standards of conduct in our trials.”

The UK drugmaker, which is licensing the vaccine candidate from the University of Oxford, began the 30,000-patient U.S. phase 3 trial on Aug. 31 at 62 sites, according to ClinicalTrials.gov.