COVID-19 patients face a much higher risk of developing blood clots than those vaccinated with AstraZeneca PLC or Pfizer Inc.’s shots, according to a large U.K. study.

For every 10 million people who receive the first dose of AstraZeneca, about 66 more will suffer from a blood-clotting syndrome than during normal circumstances, according to the study published in the British Medical Journal. This figure compares with 12,614 more incidences recorded in 10 million people who have tested positive for COVID-19.

The study followed 29 million people who received their first doses of either the AstraZeneca or Pfizer vaccine between December 2020 and April, and also tracked about 1.7 million COVID-19 patients.