A spokesperson for AstraZeneca, a frontrunner in the race for a Covid-19 vaccine, told Statnews in a statement that the company’s “standard review process triggered a pause to vaccination to allow review of safety data.”  

In a follow-up statement, AstraZeneca said it initiated the study hold. The nature of the adverse reaction and when it happened were not immediately known, though the participant is expected to recover, according to an individual familiar with the matter, reported Stat News.

The spokesperson described the pause as “a routine action which has to happen whenever there is a potentially unexplained illness in one of the trials, while it is investigated, ensuring we maintain the integrity of the trials.”

The spokesperson also said that the company is “working to expedite the review of the single event to minimize any potential impact on the trial timeline.”

An individual familiar with the development told Stat News that researchers had been told the hold was placed on the trial out of “an abundance of caution.”  

A second individual familiar with the matter, who also spoke on condition of anonymity, told the news portal the finding is having an impact on other AstraZeneca vaccine trials underway — as well as on the clinical trials being conducted by other vaccine manufacturers.