European researchers claim to have discovered a link between the AstraZeneca coronavirus vaccine and blood clots caused by rare thrombosis in the brain triggered by the mRNA vaccine in a small number of patients.

After a short period of panic, when 37 European patients experienced clotting or severe bleeding, multiple EU countries either halted vaccine distribution or limited it to specific vaccine batches; the European Union’s top pharmaceutical regulator stated that the vaccine was safe. In a similar tone to the World Health Organization’s stance on the matter, the European Medicines Agency’s executive director said that “Its benefits in protecting people from Covid-19 with the associated risks of deaths and hospitalizations outweigh the possible risks.”

The United States has not yet approved the AstraZeneca vaccine, despite ordering millions of doses ahead of time. The possibility of blood clots, while unlikely, could potentially throw a wrench in the advancement of US vaccine administration acceleration.