(Bloomberg) -- The European Union’s drug regulators said there’s a possible link between AstraZeneca Plc’s Covid-19 vaccine and a rare type of blood clot in the brain, which may increase public concern and undermine governments’ efforts to inoculate people quickly.

“Unusual blood clots with low blood platelets should be listed as very rare side effects” of the vaccine, the European Medicines Agency said Wednesday. It repeated that the overall benefits of the shot outweigh the risks.

The warning could threaten the European Union’s vaccination campaign, which was just beginning to kick into a higher gear after a sluggish start.

The latest assessment of the Astra vaccine, which was developed with the University of Oxford, comes amid increasing public wariness about the shot, with patients from Bucharest to Calais canceling scheduled appointments as they await more clarity.

©2021 Bloomberg L.P.