Blood clot risk greater after Covid infection than after vaccination

According to the largest study of its type, the risk of developing blood clots from Covid-19 virus infection far outweighs those of vaccines AstraZeneca or Pfizer.
This comprehensive analysis used data from over 29 million people in England to evaluate both vaccines against infection by Sars-Cov-2. The analysis compared hospital admissions and deaths from blood clots within 28 days of receiving either the first jab or a positive test.

Julia HippisleyCox, a professor of clinical epidemiology at the University of Oxford, was the lead author. She said that people should be aware of the increased risks associated with Covid-19 vaccination. They should seek medical attention immediately if they experience symptoms. However, they also need to be aware of the fact that Sars Cov-2 infection can cause serious complications.

These findings were based upon data from electronic health records that were collected between 24 April 2021 and 1 December 2020. The researchers looked at other risk factors, such as CVST (blood clots within the brain) or ischaemic stroke (a blood blockage or clot that stops blood supply to the brain).

They found an increase in the risk of blood clots in the veins due to thrombocytopenia and other rare blood clots following the first dose of Oxford/AstraZeneca. They found an increased risk of blood clots in the arteries and stroke after the first Pfizer/BioNTech vaccination.

The data revealed that 934 additional cases of thrombocytopenia would occur for every 10,000,000 people after infection. This compares to 107 after the AstraZeneca jab. There would be 1,699 additional cases of ischaemic strokes for every 10,000,000 people after infection. However, the number would only be 143 after the first Pfizer jab.

These conditions can occur even if there is no vaccination or infection. These risks are not related to the vaccinations or infections. Researchers cautioned that the risks associated with vaccines are short-lived in British Medical Journal.

Professor Carol Coupland, a senior researcher at Oxford and professor of primary care medical statistics at Nottingham, stated that stroke risk after Pfizer jabs was elevated for 15-21 days. The risk of developing thrombocytopenia following the AstraZeneca jab was elevated for between 8 and 14 days. She added that the association with infection was generally seen for a 28-day period following the infection.

A few other studies have shown that the AstraZeneca vaccine can increase the risk of blood clots and thrombocytopenia in those under 50, particularly in the younger population. These risks have led to changes in how the vaccine is recommended for deployment in certain countries, including the UK. Researchers said that the study was not able to simultaneously assess the risk from both conditions.

One final calculation remains to be done. It concerns whether vaccine risks are different between vaccines. Dr Peter English, a former consultant in communicable diseases control and chairman of the BMAs Public Health Medicine Committee, was not part of the study.

He said that these adverse events are rare and difficult to determine their frequency following specific vaccines. We will be able to compare more data and identify vaccines that are preferred by different recipients (age, gender, etc.) with greater confidence as we collect more data.