Risk of rare blood clots higher after Covid-19 infection than after vaccination: Study

Researchers at Oxford University have found the risk of a rare type of blood clot is low overall, but higher for people who have been infected with Covid-19 than among people who've had the three vaccines authorized in the UK - those made by AstraZeneca, Moderna and Pfizer.

According to CNN, the study, made available in pre-print on Thursday on the Oxford website ahead of publication in a scientific journal, says the risk of cerebral venous thrombosis or CVT - also known as cerebral venous sinus thrombosis or CVST - following Covid-19 infection is around "100 times greater than normal and several times higher than it is post-vaccination or following influenza," across all age groups.

As per an official statement by Oxford University, the study authors, led by Professor Paul Harrison and Dr Maxime Taquet from Oxford University's Department of Psychiatry and the NIHR Oxford Health Biomedical Research Centre, counted the number of CVT cases diagnosed in the two weeks following diagnosis of COVID-19, or after the first dose of a vaccine.

The then compared these to calculated incidences of CVT following influenza, and the background level in the general population.

They report that CVT is more common after COVID-19 than in any of the comparison groups, with 30% of these cases occurring in the under 30s. Compared to the current COVID-19 vaccines, this risk is between 8-10 times higher, and compared to the baseline, approximately 100 times higher.

In this study of over 500,000 COVID-19 patients, CVT occurred in 39 in a million patients. In over 480,000 people receiving a COVID-19 mRNA vaccine (Pfizer or Moderna), CVT occurred in 4 in a million. CVT has been reported to occur in about 5 in a million people after the first dose of the AZ-Oxford COVID-19 vaccine.

The study found that compared to the mRNA vaccines, the risk of a CVT from COVID-19 is about 10 times greater. Whereas, compared to the AZ-Oxford vaccine, the risk of a CVT from COVID-19 is about 8 times greater.

Paul Harrison, Professor of Psychiatry and Head of the Translational Neurobiology Group at the University of Oxford, said in a statement: "There are concerns about possible associations between vaccines, and CVT, causing governments and regulators to restrict the use of certain vaccines. Yet, one key question remained unknown: 'What is the risk of CVT following a diagnosis of COVID-19?"

"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 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," he added.

As per the official statement, an important factor that requires further research is whether COVID-19 and vaccines lead to CVT by the same or different mechsms. There may also be under-reporting or mis-coding of CVT in medical records, and therefore uncertainty as to the precision of the results.

According to CNN, this comes as European and British medicines regulators last week announced a "possible link" between the AstraZeneca vaccine and rare cases of blood clots, with the UK announcing it would offer people under 30 an alternative vaccine. Other countries have followed suit and are either only offering to people above a certain age or are like Denmark and Norway, scrapping the vaccine entirely.

While advising the public to look out for the signs of clots, the regulators said the benefits of the shot were still worth the risk. The AstraZeneca vaccine has not been authorized for use in the United States.

Six reports of similar clotting events following vaccination with the single-dose Johnson & Johnson vaccine prompted the US Centers for Disease Control and Prevention and the US Food and Drug Administration to recommend a pause on administering the vaccine to allow for further investigation.

Six women between the ages of 18 and 48 had developed a cerebral venous sinus thrombosis, a clot in the area of the brain that collects and drains oxygen-depleted blood. Blood thinners, the typical treatment for clots, should not be used in such cases. The six reported cases were among more than 6.8 million doses of the Johnson & Johnson vaccine administered in the United States.

The WHO on Thursday said "for now the risk of suffering blood clots, is much higher for someone with COVID-19 than for someone who has taken the AstraZeneca vaccine." WHO Regional Director for Europe Hans Kluge reiterated its recommendation of the AstraZeneca vaccine for all eligible adults, calling it "effective in reducing COVID-19 hospitalization and preventing deaths."

However, US infectious diseases expert Anthony Fauci said that there remains some confusion around new research from the University of Oxford in the United Kingdom that compares the risk of a rare type of blood clot among people who have had Covid-19 with people who received the AstraZeneca, Moderna and Pfizer vaccines.

Fauci who is director of the National Institute of Allergy and Infectious Diseases made the comments during a hearing with the House Select Subcommittee on the Coronavirus Crisis Thursday.

"They were trying to find out the difference in the incidence of thromboses, particularly cerebral venous thromboses, following the disease Covid-19 compared to various vaccinations, including influenza as well as the mRNA vaccines of Pfizer as well as Moderna," Fauci said.

"They found that - as you might expect - following the disease, you get a very marked increase in the incidence of this adverse situation of cerebral venous thrombosis."

However, Fauci added that when the researchers calculated what the incidence of these thromboses may be following Covid-19 vaccination to compare incidents following different types of vaccines, some concerns in the methodology emerged.

"It is impossible, the way this study was designed and conducted to make that determination. So, I believe when this paper, which is in a pre-print server, gets submitted to the classical scientific journals and undergoes peer review that that confusion will be straightened out," Fauci said.

( With inputs from ANI )