EU medicine regulator's review of the Oxford-AstraZeneca vaccine has concluded that "unusual blood clots with low blood platelets should be listed as very rare side effects".
"The reported combination of blood clots and low blood platelets is very rare, and the overall benefits of the vaccine in preventing Covid-19 outweigh the risks of side effects," the European Medicines Agency (EMA) said, reports the BBC.
Most of the cases reported had occurred in women under 60 years of age within two weeks of vaccination.
The regulator said that one plausible explanation for the combination of blood clots and low blood platelets was an "immune response, leading to a condition similar to one seen sometimes in patients treated with heparin (heparin induced thrombocytopenia, HIT)".
The EMA's Emer Cooke said that clots were seen in "all ages, and in men and women", and there was no available evidence of "specific risk factors such as age, gender, or previous medical history of clotting disorders".
"Our safety committee... has confirmed that the benefits of the AstraZeneca vaccine in preventing Covid-19 overall outweigh the risks of side effects," she said.
"Covid-19 is a very serious disease with high hospitalisation and death rates and every day Covid is still causing thousands of deaths across the EU.
"This vaccine has proven to be highly effective - it prevents severe disease and hospitalisation, and it is saving lives."