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A large study from Denmark and Norway, published today by the BMJ, sheds light on the risk of rare blood clots in adults who receive their first dose of the Oxford-AstraZeneca COVID-19 vaccine.
The results show slightly increased rates of venous blood clots, including clots in the veins of the brain, compared to the expected rates in the general population. However, the researchers emphasize that the risk of such adverse events is considered to be low.
Cases of rare blood clots in people who recently received their first dose of the Oxford-AstraZeneca COVID-19 vaccine have been reported. However, it has been debated whether these cases represent overevents that are above expected rates in the general population.
Both UK and European drug regulators say the benefits of the Oxford-AstraZeneca vaccine still outweigh the risks. Denmark and Norway have suspended use of the vaccine, and several other countries have decided to suspend use in certain age groups.
To investigate this further, researchers in Denmark and Norway attempted to compare nationwide rates of blood clots and related diseases after vaccination with the Oxford-AstraZeneca vaccine with those in the general population in both countries.
Their results are based on 280,000 people aged 18 to 65 who received a first dose of the Oxford-AstraZeneca COVID-19 vaccine from February 2021 to March 11, 2021 in Denmark and Norway.
Using national health records, they identified event rates such as myocardial infarction, stroke, deep vein clots and bleeding events within 28 days of receiving an initial vaccine dose and compared these to expected rates in the general population of Denmark and Norway.
In the main analysis, the researchers found 59 blood clots in the veins compared with 30 expected, which equates to 11 excess events per 100,000 vaccinations. This included a higher than expected rate of blood clots in the veins of the brain known as cerebral vein thrombosis (2.5 events per 100,000 vaccinations).
However, they did not find an increase in the rate of arterial blood clots such as heart attacks or strokes.
For most of the remaining results, the results were largely reassuring, with slightly higher rates of less severe events such as thrombocytopenia (a condition associated with low platelet levels), bleeding disorders, and bleeding, which they believed could be influenced by increased surveillance of vaccine recipients.
This is an observational study, so no cause can be identified, only a correlation. And the researchers point out some limitations, like the lack of data on the underlying risk factors for clotting and the possibility that their results may not apply to other races.
However, its strengths include the population-based approach, which uses reliable national registration data and virtually all of the participants are tracked.
They conclude that "the absolute risks of venous thromboembolic events described in this study are low and the results should be interpreted in the context of the benefits of COVID-19 vaccination at both a societal and an individual level".
In a linked editorial, BMJ editors Rafael Perera and John Fletcher point out that COVID-19 is itself associated with cerebral vein thrombosis, saying that vaccination remains largely the safest option.
"We almost all have a choice between possible SARS CoV2 infection or vaccination. The Astra-Zeneca vaccine is clearly a good choice, despite the likely risks reported in this study," they write.
"Quantifying the comparative risk associated with other vaccines is now a research (and public health) priority," they conclude.
In a linked statement, Professor Paul Hunter of the University of East Anglia says this study improves our understanding of the population risk of thrombotic events following the Oxford-AstraZeneca vaccine, but does not change the conclusions of the UK and European drug regulators about the benefits of the Oxford-AstraZeneca – Vaccine far outweigh the risks for most age groups.
"Countries that delayed their own immunization programs at a time of high transmission rates by rejecting the use of available Oxford-AstraZeneca vaccines should know that their decision has contributed to an increase in preventable deaths from COVID-19," he argues.
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Arterial events, venous thromboembolism, thrombocytopenia and bleeding after vaccination with Oxford-AstraZeneca ChAdOx1 in Denmark and Norway: population-based cohort study, www.bmj.com/content/373/bmj.n1114
Opinion: Thrombosis and bleeding after the Covid-19 vaccination against Oxford-AstraZeneca, blogs.bmj.com/bmj/2021/05/05/t… covid-19 vaccination
British Medical Journal
The study sheds light on the rate of rare blood clots following the vaccine against Oxford-AstraZeneca (May 5, 2021).
accessed on May 5, 2021
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