Question: Why do vaccines clot your blood

Toby Bonvoisin answered on 4 Feb 2022:

Blood clotting as a vaccine side effect is incredibly rare. Everyone’s immune systems are different, and somewhere between one in 100,000 and one in a million people have an immune system that seems to react strangely to the Oxford-AstraZeneca vaccine. In these people the immune system triggers clotting in the blood vessels in the brain.

Covid-19, however, can also cause clotting in the brain and elsewhere in the body, particularly the lungs. Clots in the blood can also happen for no obvious reason at all.

For young people, because their risk of severe disease with covid is very low, the risk of blood clots in the brain from the AstraZeneca vaccine is more than the risk of blood clots from covid itself. This is why, in the UK, young people are offered other vaccines such as Pfizer and Moderna that do not pose this risk but are also very good at protecting you from covid.