MEDICAL ADVICE IS that there is a higher risk of blood clots from some commonly-used drugs and that the statistical likelihood of being struck by lightning is greater than of getting a blood clot from COVID-19 vaccination. Now a new study suggests getting the disease itself poses a higher risk factor for brain blood clots than risk from a vaccine. The Australian Science Media Centre explains with some comment from Australian experts. “The risk of rare blood clotting known as cerebral venous thrombosis (CVT) following COVID-19 infection is around 100 times greater than normal, and several times higher than it is post-vaccination or following influenza, according to research based on US data. “The research has not been published in a journal and is yet to be peer-reviewed. The authors report that CVT is more common after COVID-19 than after the first dose of an mRNA COVID-19 vaccine, or following influenza. The authors then also refer to the risk of CVT following the AstraZeneca vaccine using data from the European Medicines Agency and say the risk of CVT from COVID-19 infection is also higher than the rate reported for this vaccine.
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Dr Daryl Cheng the Medical Lead of the Melbourne Vaccine Education Centre noted:
“The study from Oxford University aims to estimate how common thromboses were following COVID-19 disease, in particular cerebral venous thromboses (CVT) and portal vein thromboses (PVT). They then compared these rates with the incidence of CVT and PVT in populations receiving mRNA (Pfizer and Moderna) or COVID-19 AstraZeneca vaccines.
“The study finds a significant increase in the rate of thromboses after COVID-19 disease, which corresponds with existing data showing that COVID-19 has a procoagulant or pro-clotting impact in both adults and children. According to the study, there seems to be a lower rate of these same clots in people who have had either a mRNA or viral vector COVID-19 AstraZeneca vaccine.”
Dr Cheng said there are still unanswered questions and challenges from the research method that used data sources from different populations and countries and are not directly comparable and they were not matched for age or other confounding factors. He also noted that in a country with a low incidence of COVID-19 like Australia while the comparative risks are less, effective vaccine strategies remain vital regardless, to lower the medical burden and should inform individual decisions.
Dr Paul Griffin Director of Infectious Diseases at Mater Health Services, Associate Professor of Medicine at the University of Queensland, added more detail.
“This is a large study utilising electronic health records that includes over 500,000 COVID-19 diagnoses, nearly 200,000 influenza diagnoses and nearly 500,000 recipients of mRNA vaccines.
“The reported incidence of CVT following COVID-19 diagnosis was 30.0 per million people, which is significantly higher than the rates seen following influenza of 0.0 per million people and following receipt of mRNA vaccine at 4.1 per million people.
“Despite some limitations this is an interesting study in so far as that it shows there are potentially other causes of CVT including COVID-19 itself and highlights that when we are making assessments of the risks versus benefits of vaccines in the context of adverse events we also have to consider other causes of these events including the infection itself as well as the background risks of such events.”
(Full disclosure — Paul is on the AstraZeneca advisory board and is also trialling other COVID vaccines including Novavax.)
Dr Indu Singh is an Assoc Professor of Haematology and Program Director of the Bachelor of Medical Laboratory Science at Griffith University summarised :
“Blood clotting leading to thrombosis following COVID-19 infection is 8–10 times higher than seen in cases after vaccination. In the UK, 39 in a million COVID-19 patients presented with thrombosis, while post-COVID-19 mRNA vaccine (Pfizer or Moderna), the rate was 4 in a million and post AstraZeneca vaccine, 5 in a million people were impacted.
“There is some risk of blood clotting post any COVID-19 vaccination but that is much lower than the rate after getting COVID infection. It has been seen that the risk of thrombosis from COVID-19 is about 10 times greater than after mRNA vaccines and 8 times greater than after DNA vaccine AstraZeneca vaccines.
“This data and similar data from Europe show balance between risks and benefits of vaccination should be considered.
“Currently, there is insufficient evidence suggesting vaccines are directly responsible for thrombosis. In rare patients, an unusual immune response may be triggered by vaccination leading to blood clots. It is possible people impacted were predisposed to a clotting issue and the vaccine triggered it.
“I believe vaccinations should continue in regions with community transmission of the virus. More evidence is required, and quickly, to make a conclusive statement but in the meantime, some protection is better than nothing.”
[*Cerebral venous thrombosis: a retrospective cohort study of 513,284 confirmed COVID-19 cases and a comparison with 489,871 people receiving a COVID-19 mRNA vaccine, Taquet et al., preprint, https://osf.io/a9jdq/ ]
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