A new University of Oxford study, which has yet to be peer reviewed, looked at CVT cases diagnosed in the two weeks following a diagnosis of COVID-19 and two weeks after a first dose of a vaccine.
The European Medicine Agency (EMA) has estimated that the risk of CVT occurring in someone who has received a first dose of the Oxford/AstraZeneca vaccine is 5 per million people. The link between the vaccine and the rare side effect prompted the MHRA to recommend that those aged under 30 should be offered an alternative jab last week following a risk/benefit analysis.
This latest research looked at data primarily from the US. It found that in over 500,000 patients who had COVID-19, CVT occurred in 39 in a million patients.
In over 480,000 people receiving a COVID-19 mRNA vaccine (Pfizer/BioNTech or Moderna), CVT occurred in 4 in a million people.
The baseline incidence of CVT in the general population across any two-week period was 0.41 per million patients.
Increased risk of CVD after COVID-19
The researchers said the risk of CVT from COVID-19 infection was eight times greater when compared with the Oxford/AstraZeneca vaccine and ten times greater when compared with the mRNA vaccines.
Infection with COVID-19 meant the risk of of CVT was increased 100-fold compared with the general population rate.
The researchers also separately looked at portal vein thrombosis and found the risk was significantly higher in the two weeks after a diagnosis of COVID-19, than after vaccination with an mRNA vaccine. They found an incidence of 436.4 per million people following infection compared with 44.9 per million people following vaccination.
The researchers said that the comparisons should be interpreted cautiously because data were still being gathered. They also highlighted that the data on the Oxford/AstraZeneca vaccine came from the EMA monitoring system, while other data was based on findings mainly from the US, and that there could be issues with under-reporting or coding in the health records.
Risks and benefits of vaccination
However, Professor Paul Harrison, professor of psychiatry and head of the Translational Neurobiology Group at the University of Oxford and one of the study's lead authors, said the research had reached two 'important conclusions'.
He said: 'Firstly, COVID-19 markedly increases the risk of CVT, adding to the list of blood clotting problems this infection causes. Secondly, the COVID-19 risk is higher than see with the current vaccines, even for those under 30; something that should be taken into account when considering the balances between risks and benefits for vaccination.’
The researchers said that further research was needed to esstablish whether the same mechanism was behind the CVT that occured in those with COVID-19 infection and those who are vaccinated.