The risk of contracting blood clots is much higher for Covid-19 patients than from vaccines, according to a study released on Thursday.
The rare blood clotting known as cerebral venous thrombosis, or CVT, occurred at a rate of 39 per million Covid 19 patients, according to researchers at the University of Oxford, the same university that helped develop the AstraZeneca vaccine. That compares to five in a million people after the first dose of the AstraZeneca-Oxford vaccine, they said.
“We’ve reached two important conclusions. 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 we 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,” said Paul Harrison, professor of psychiatry and head of the translational neurobiology group at the University of Oxford.
Using a U.S. health records network, the study compared more than 500,000 patients with Covid-19 to nearly 200,000 with influenza and another roughly 500,000 who received the mRNA vaccine. They also drew on European Medicines Agency data of those who took the
vaccine. “…all comparisons must be interpreted cautiously since data are still accruing,” according to the study.
The researchers claimed they found the rare blood clotting at a rate of four out of a million people receiving the vaccines from drug company
and biotech Moderna, but Pfizer disputed that claim. In a statement, Pfizer said its own review of safety data after over 200 million doses have been administered has found no evidence to conclude that arterial or venous thromboembolic events being investigated in the Johnson & Johnson vaccine are a risk associated with its Covid-19 vaccine.
“CDC reported that no similar findings have been observed with the authorized Pfizer-BioNTech vaccine,” the statement said. “With a vast number of people vaccinated to date, no medical observations have modified the safety profile of our vaccine observed in phase 3 clinical trials.” Moderna was not immediately available for comment.
Both the Pfizer and Moderna vaccines are produced with the mRNA process, which uses molecules in cells that control protein production to teach the immune system to make coronavirus-fighting antibodies. The U.S. is now reviewing the vaccine from pharmaceutical Johnson & Johnson, which is made using a similar process as the AstraZeneca vaccine and also has triggered blood-clot concerns.