No idea.
Both my parents got the vax. No problems whatsoever. They’re in their 70s and my father has had multiple heart problems so he’s on thinners. The reason why it’s so dangerous from 50 years and below is because of the clotting and sudden immune attacks on the heart. Older folks don’t have very active immune systems and many of them ARE on blood thinners.
That was my guess as well. I am in my seventies and have had no issues with the vaccines.