AstraZeneca shares plunged more than 7% in after-hours trading following news that the company's Phase 3 trial for its COVID-19 vaccine has been put on hold due to safety concerns.
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A person familiar with the development told STAT News that researchers were told the hold was placed on the trial out of “an abundance of caution” after a suspected serious adverse reaction in a participant in the United Kingdom.
AstraZeneca is investigating to determine whether the adverse reaction is linked to the vaccine.
"As part of the ongoing randomized, controlled global trials of the Oxford coronavirus vaccine, our standard review process was triggered and we voluntarily paused vaccination to allow review of safety data by an independent committee," a spokesperson for AstraZeneca told FOX Business in a statement. "This is 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. In large trials illnesses will happen by chance but must be independently reviewed to check this carefully."
The company noted that it's "working to expedite the review of the single event to minimize any potential impact on the trial timeline."
"We are committed to the safety of our participants and the highest standards of conduct in our trials," the spokesperson added.
While temporary holds on vaccine trials are not uncommon, it’s unclear how long the hold on the trial will last.
The vaccine, known as AZD1222, is being developed with the University of Oxford. AstraZeneca began recruiting 30,000 people for the latest trial in the U.S. last month. The vaccine is also being tested in thousands of people in Britain, and in smaller studies in Brazil and South Africa.
A Phase 1/2 study published in July reported about 60% of 1,000 participants on the vaccine experienced side effects.
Meanwhile, vaccine candidates developed by Moderna and a partnership between Pfizer and BioNTech are both in final-stage tests in the United States that have already recruited about two-thirds of the needed volunteers.
The Associated Press contributed to this report