Link to report (in German) : https://gth-online.org/wp-content/uploads/2021/03/GTH_Stellungnahme_AstraZeneca_3_19032021-3.pdf
There was not previously any evidence of a clotting problem with the vaccine in the phase 3 trial. It actually found higher rates of blood clots in the placebo group than the vaccine group. However, the clots that raised concern in Europe were not the relatively common blood clots that can form in your legs if you sit for too long. They were clots in the large veins of the brain (cerebral vein thrombosis). This is much rarer.
In Germany, there were 13 cases of this type of clot after about 1.6 million vaccinations. That's one case in 123,000 vaccinations, so way too rare to be picked up by the phase 3 trial. All but one were in women. They were also fairly young. A group of scientists in Germany have released a statement saying that they had found evidence of something similar to a rare autoimmune disorder called heparin-induced thrombocytopenia (HIT) in four of these patients.
Thrombocytopenia means that your platelet levels are too low. The heparin-induced part is due to antibodies that bind "backwards" (stem-first) to the platelets and activate them. This is different from autoimmune thrombocytopenia (ATP), where the antibodies bind to platelets through the arms rather than the stem. HIT causes excess clots; ATP causes excess bleeding.
In HIT, the activation of platelets by the antibodies is triggered by heparin. Heparin is an anticoagulant, so you can end up with someone having a small clot, seeking medical attention, getting heparin as a treatment, and having an even worse clot in response. If the person treating the patient knows that the problem is HIT, they can choose a different anticoagulant to treat the clot, and the researchers in Germany are recommending that doctors in locations where this vaccine is being given be aware of this potential problem.
HIT is not a long-term disease. Patients who survive are expected to make a full recovery.
The researchers emphasize that they don't know that these cases of HIT were caused by the vaccine. I can't find a background rate relative to the total population for HIT. The studies I'm finding are looking at what percentage of people given heparin develop complications from HIT.
Looking for rates of cerebral vein thrombosis, a Dutch study found a background rate of 1.3 per 100,000 per year, so in a 16 day window after the vaccine, the background rate would be expected to be 0.06 per 100,000. So just based on that, it looks like the roughly 0.8 per 100,000 seen in Germany is a signal well above background. Both HIT and cerebral vein thrombosis are more common in women than men, although not 12 to 1.
Even so, a rate of 0.8 per 100,000 is still pretty rare, so I don't think it makes sense to pull the vaccine on this news. It's also good news for people who are prone to blood clots for reasons totally unrelated to autoimmunity. People who are prone to antibody-based autoimmunity might want to consider going with one of the other vaccine options.
If you have the time/interest, consider this CVT study for extra reading: https://www.ahajournals.org/doi/10.1161/strokeaha.112.671453