Why some covid vaccines caused clotting

[shpalman]

When COVID-19 vaccines from AstraZeneca and J&J rolled out in 2021, a small number of people developed dangerous blood clots combined with low platelets and bleeding. It was rare (about 1 in 200,000 vaccinated people) but serious.

Both vaccines used an adenovirus as a delivery vehicle. Researchers found that natural adenovirus infections can sometimes cause a similar syndrome.

It's because the adenovirus carries a protein that resembles a human protein involved in clotting. When the immune system encounters it, it makes antibodies to fight it. Up to 60% of people have a common inherited variation in how those antibodies are shaped, making them susceptible to confusing the two proteins. A second thing also has to happen: a very specific mutation must occur within that same antibody-producing B cell that alters the binding properties of the antibody, resulting in ‘redirection’ to the body’s own clotting factor instead of the virus protein. Both things must happen in the same B cell in the same person, which is why this was so rare, and why it was never seen with mRNA vaccines, which use completely different technology with no adenovirus involved.

https://www.nature.com/articles/d41586-026-00457-4