COVID-19 presents greater blood clot risk than vaccines, study finds

Blood clot risk high for COVID-19 patients: study A new U.K. study suggests COVID-19 patients face a much higher risk of developing blood clots than those vaccinated with AstraZeneca or Pfizer shots.

TORONTO — The risk of developing blood clots is substantially higher and more prolonged if you contract COVID-19 compared to receiving the Oxford-AstraZeneca or Pfizer-BioNTech vaccines, according to a new study.

The study found that while there is a small increased risk of potentially deadly clots for a short time after receiving a dose of the AstraZeneca vaccine, the chances of such adverse events are nearly 200 times higher if one becomes infected with the virus.

"People should be aware of these increased risks after COVID-19 vaccination and seek medical attention promptly if they develop symptoms, but also be aware that the risks are considerably higher and over longer periods of time if they become infected with SARS-CoV-2," Julia Hippisley-Cox, professor of clinical epidemiology and general practice at the University of Oxford and lead author of the study, said in a press release.

The study, published Friday in the British Medical Journal, analyzed the health data of over 29 million people in the U.K. within 28 days of receiving their first COVID-19 vaccine dose between Dec. 1, 2020 and April 24, 2021. Approximately 19.5 million received an AstraZeneca dose and 9.5 million a Pfizer shot. Data from these individuals were compared with those from about 1.75 million patients who tested positive for COVID-19 during the same period.

Overall, researchers found the risk of blood clots, low platelet counts and stroke were much higher and more prolonged upon infection with COVID-19 than after receiving a dose of either vaccine.

Researchers estimated that for every 10 million COVID-19 cases or vaccine inoculations, there would be 66 additional cases of blood clots in the veins after a dose of the AstraZeneca vaccine compared to 12,614 among those infected with the virus.

AstraZeneca doses could introduce 107 additional cases of low platelet counts compared to 934 from those with the virus, while the Pfizer vaccine could result in 143 additional cases of stroke compared to 1,699 instances among those with COVID-19.

Jeffrey Weitz, executive director of the Thrombosis and Atherosclerosis Research Institute at McMaster University in Hamilton, Ont., said the risk of clotting after the first dose of a COVID-19 vaccine remains small.

"I think this information should just inform individuals that they need to get vaccinated," Weitz told CTV News Channel on Saturday. "[Compared] to not being vaccinated, your risk of getting a blood clot is at least 10 to 100 times higher."

Many provinces across Canada suspended rollouts of the AstraZeneca vaccine earlier this year due to reports of an increased risk of rare but serious blood clots associated with low levels of platelets.