Blood clot risk greater after Covid infection than after vaccination
Analysis of 29m people finds danger of infection with Sars-Cov-2 far outweighs the risks of having jab
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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.
Lead author Julia Hippisley-Cox, professor of clinical epidemiology and general practice at the University of Oxford, said: “People should be aware of these increased risks after Covid-19 vaccination and seek medical attention promptly if they develop symptoms, but also be aware that the risks are considerably higher and over longer periods of time if they become infected with Sars Cov-2.”
The findings were based on data from electronic health records collected between 1 December 2020 and 24 April 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.
People can develop these conditions even without infection or vaccination. The risks described in the study are the additional risks brought by the vaccines or infection. The risks associated with the vaccines were also relatively short-lived, the researchers cautioned in the British Medical Journal.
Prof Carol Coupland, senior researcher at the University of Oxford and professor of medical statistics in primary care at the University of Nottingham, said that the stroke risk with the Pfizer jab was elevated for 15 to 21 days after the first dose. In the case of thrombocytopenia after the AstraZeneca jab, that risk was higher for 8 to 14 days. “Whereas the associations with infection appeared to be generally over a whole 28-day period after the infection,” she added
Other smaller studies have linked the risk of thrombocytopenia alongside blood clots to after the first dose of the AstraZeneca vaccine, in particular in people under the age of 50. Such risks have triggered changes in the way the vaccine has been recommended for deployment in some countries, including the UK. This study, however, was not powered to assess the risk of both conditions concurrently, the researchers said.
“There is one final remaining calculation to be made, and that relates to whether the risks differ between vaccines,” said Dr Peter English, a retired consultant in communicable disease control and former chairman of the BMA’s public health medicine committee, who was not involved in the study.
He added: “The rarity of these adverse events makes it difficult to quantify precisely their frequency after specific vaccines. As we accumulate more data, we will become more confident in our comparisons; and it may be that this will enable us to identify which vaccines (if any) are to be preferred in different categories (age, sex, etc) of recipient with increased confidence.”