The U.S. Food and Drug Administration is broadening its probe into a serious illness tied to AstraZeneca's COVID-19 vaccine study, according to three sources familiar with the details.
Concerns about the AstraZeneca vaccine stem from a study participant in Britain who came down with what was believed to be a rare spinal inflammatory disorder.
Sources tell Reuters that the FDA is looking at AstraZeneca's development partner Oxford University to see if other vaccines designed by the duo resulted in similar side effects.
Trials in the U.S. have been put on hold since early September.
But not in Europe. On Thursday, the region's health regulator said it has started a rolling review of the AstraZeneca vaccine in a move which could speed the approval process for any vaccine.
AstraZeneca faces a different scenario in the U.S. With the FDA widening its investigation, that presents a risk there could be additional delays for what has been one of the leading frontrunners in the race to get a vaccine to market.
One source said the FDA is doing its due diligence to show it is being thorough.
The FDA declined to comment and AstraZeneca said in a statement: "We are continuing to work with the FDA."
Drug companies are under public scrutiny amid worries about the safety of a vaccine developed and tested at an unprecedented speed. To counter that, nine drugmakers including AstraZeneca have pledged to uphold scientific safety and efficacy standards.
Finding an effective vaccine would be a major step towards ending the pandemic that has killed more than one million people worldwide.