Europe’s drug regulator has found a possible link between AstraZeneca’s Covid-19 vaccine and rare blood clotting issues in adults who had received the shot and said it had taken into consideration all available evidence.

“One plausible explanation for the combination of blood clots and low blood platelets is an immune response, leading to a condition similar to one seen sometimes in patients treated with heparin,” the European Medicines Agency said.

The findings come as a major hurdle in the global fight against the pandemic and a shift in the stance of the regulator, which had last week backed the vaccine and said there was no increased risk of blood clots in general from the shot.