New Delhi: Blood clots and the fear of developing them are being talked about ever since the pandemic began. Now, it turns out that the risk of developing blood clots is considerably higher after infection with the SARS-CoV-2 virus than after immunisation with Pfizer and AstraZeneca COVID-19 vaccines, according to a large UK study published in the British Medical Journal (BMJ) on Friday.
The team led by researchers at the University of Oxford in the UK conducted the study on thrombocytopenia, a condition with low platelet counts and thromboembolic events or blood clots following COVID-19 vaccination with Oxford-AstraZeneca and Pfizer-BioNTech preventive, PTI reported.
Some of these events have led to restricted use of the Oxford-AstraZeneca vaccine, known as Covishield in India, in a number of countries. The researchers compared rates of adverse events after vaccination with rates of the same events after a positive SARS-CoV-2 test result using data from over 29 million people in the UK who received the first dose of either of the two vaccines.
They concluded that with both of these vaccines, for short time intervals following the first dose, there are increased risks of some blood-related adverse events leading to hospitalisation or death.
“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,” said Julia Hippisley-Cox, professor at the University of Oxford, and lead author of the research paper.
The authors noted that the risk of these adverse events is substantially higher and for a longer period of time, following infection from the SARS-CoV-2 coronavirus than after either vaccine.
All of the coronavirus vaccines currently in use have been tested in randomised clinical trials, which are unlikely to be large enough to detect very rare adverse events, they said.
When rare events are uncovered, then regulators perform a risk-benefit analysis of the medicine, to compare the risks of the adverse events if vaccinated versus the benefits of avoidance of the disease — in this case, COVID-19.
The authors pointed out some limitations to their study, including restricting the analysis to first vaccine dose only, a short vaccination exposure window, and the potential for misclassification of outcomes or exposures admissions where patients were still in hospital.
However, they believe that any bias, if present, is likely to not change with respect to each vaccine, and so the comparisons between vaccines are unlikely to be affected. “This research is important as many other studies, while useful, have been limited by small numbers and potential biases,” Hippisley-Cox said.
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