AstraZeneca Plc has paused a late-stage trial of one of the leading Covid-19 vaccine candidates after a suspected serious adverse reaction in a study participant, health news website Stat News reported on Tuesday.

It quoted an AstraZeneca spokesperson as saying in a statement that the “standard review process triggered a pause to vaccination to allow review of safety data.”

The study is testing a Covid-19 vaccine being developed by AstraZeneca and University of Oxford researchers at various sites, including the United Kingdom, where the adverse event was reported.

The nature of the case and when it happened were not detailed, although the participant is expected to recover, according to Stat News.

The suspension of the trial has impacted other AstraZeneca vaccine trials – as well as clinical trials being conducted by other vaccine makers, which are looking for signs of similar reactions, Stat said.

The AstraZeneca spokesperson’s statement said that “in large trials, illnesses will happen by chance but must be independently reviewed to check this carefully.” Stat reported that serious adverse reactions vary and can include issues that require hospitalization, life-threatening illness and death.

AstraZeneca did not immediately respond to a request for comment.

Nine leading U.S. and European vaccine developers pledged on Tuesday to uphold scientific safety and efficacy standards for their experimental vaccines despite the urgency to contain the coronavirus pandemic.

The companies, including AstraZeneca, Pfizer Inc and GlaxoSmithKline, issued what they called a “historic pledge” after a rise in concern that safety standards might slip in the face of political pressure to rush out a vaccine.

The companies said they would “uphold the integrity of the scientific process as they work towards potential global regulatory filings and approvals of the first Covid-19 vaccines.”

The other signatories were Johnson & Johnson, Merck & Co, Moderna Inc, Novavax Inc, Sanofi and BioNTech.