COVID-19, not vaccination, poses greatest risk of blood clots: study

Researchers at the University of Oxford today announced the results of a study into thrombocytopenia (a disease with a low platelet count) and thromboembolic events (blood clots) following vaccination against COVID-19, some of the same events that have led to the restricted use of the Oxford-AstraZeneca vaccine in a number of countries.

Write in the British medical journal (BMJ), they detail the results of more than 29 million people vaccinated with the first doses of the ChAdOx1 nCov-19 vaccine “Oxford-AstraZeneca” or the mRNA vaccine BNT162b2 “Pfizer-BioNTech”. They conclude that with these two vaccines, for short time intervals after the first dose, there are increased risks of certain hematologic and vascular adverse events leading to hospitalization or death.

Julia Hippisley-Cox, professor of clinical epidemiology and general medicine at the University of Oxford, lead author of the article, said:

“People need to 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 are infected. by SARS-CoV-2. . ”

The authors further note that the risk of these adverse events is significantly higher and over a longer period of time after infection with the SARS-CoV-2 “coronavirus” than after either. vaccines.

All currently used coronavirus vaccines have been tested in randomized clinical trials, which are unlikely to be large enough to detect very rare adverse events. When rare events are discovered, regulators perform a risk-benefit analysis of the drug; compare the risks of adverse events if vaccinated against the benefits of avoiding the disease – in this case, COVID-19.

In this article, the team of authors from the University of Oxford, the University of Leicester, the Guys and St Thomas’ NHS Foundation Trust, the Intensive Care National Audit & Research Center, the London School of Hygiene and Tropical Medicine, from the University of Cambridge, the University of Edinburgh and the University of Nottingham, compared the rates of adverse events after vaccination with the Pfizer-BioNTech and Oxford-AstraZeneca vaccines with the rates of the same events after a positive SARS-CoV-2 test result.

To do this, they used systematically collected electronic health records to assess the short-term (within 28 days) risks of hospitalization for thrombocytopenia, venous thromboembolism (VTE) and arterial thromboembolism (ATE), using data collected in all of England between December 1. 2020 and April 24, 2021. The other outcomes studied were cerebral venous sinus thrombosis (CVST), ischemic stroke, myocardial infarction and other rare arterial thrombotic events.

Professor Hippisley-Cox added:

“This research is important because many other studies, while useful, have been limited by small numbers and potential bias. Electronic health records, which contain a detailed record of vaccinations, infections, results and factors of confusion, have provided us with a rich source of data with which to perform a robust assessment of these vaccines and compare them to the risks associated with COVID-19 infection. ”

The authors detail the following limitations to their study:

• restrict analysis to the first dose of vaccine only
• a short window of exposure to vaccination
• the lack of formal assessment of routinely acquired outcomes and the potential for misclassification of outcomes or exposures
• admissions where patients were still hospitalized at the end of the study were excluded.

However, they believe that any bias, if present, is unlikely to change for each vaccine and therefore comparisons between vaccines are unlikely to be affected.

Andrew Morris, Director, Health Data Research UK and Head of the National Data and Connectivity Baseline Study:

“-19 vaccines. The analyzes in this article are a vital addition to all the work HDR UK has been doing to improve our understanding of the virus, and a key result of the National Baseline Data and Connectivity Study.

Aziz Sheikh, professor of primary care research and development and director of the Usher Institute at the University of Edinburgh and co-author of the article, said:

“This huge study, using data from over 29 million people vaccinated, has shown that there is a very low risk of clotting and other blood disorders after the first dose of COVID-19 vaccination. Although serious, the risk of these same results is much higher after infection with SARS-CoV-2.

Overall, this review therefore clearly underscores the importance of getting the vaccine for reducing the risk of these clotting and bleeding results in individuals, and because of the substantial public health benefits of COVID vaccinations. -19. ”