Blood clot risk greater after Covid infection

The chances of developing dangerous blood clots after being infected with the virus that causes Covid-19 far outweighs the risks of the AstraZeneca and Pfizer vaccines, according to the largest study of its kind.

The sweeping analysis used data from more than 29 million people in England to compare both vaccines with infection from Sars-CoV-2. It weighed up rates of hospital admission or death from blood clots, as well as other blood disorders, within 28 days of either a positive test or receiving the first jab.

The findings were based on data from electronic health records collected between December 1st, 2020, and April 24th, 2021.

In addition to thrombocytopenia (a condition characterised by low platelet counts) and blood clots, the researchers also looked at certain other risks, including CVST (blood clots in the brain) and ischaemic stroke (a blood clot or blockage that cuts off the blood supply to the brain).

Overall, they found an increased risk of thrombocytopenia, blood clots in veins and other rare arterial blood clots after a first dose of the Oxford/AstraZeneca vaccine. After the first dose of the Pfizer/BioNTech vaccine, they found a higher risk of blood clots in arteries and ischaemic stroke.

However, the data showed that there would be 934 extra cases of thrombocytopenia for every 10 million people after infection, compared with 107 after the first shot of the AstraZeneca jab. For ischaemic strokes, there would be an estimated 1,699 extra cases for every 10 million people after infection, while there would be only 143 extra cases after the first Pfizer jab.