The Paul Ehrlich Institute (German Federal Institute for Vaccines) has an excellent FAQ on the topic [1].

Here is the direct link to the statement from the German Society for Thrombosis Research that pioneers the idea of heparin-induced thrombocytopenia as an allergic reaction [2] (PDF, unfortunately in German).

[1] https://www.pei.de/EN/service/faq/coronavirus/faq-coronaviru...

[2] https://gth-online.org/wp-content/uploads/2021/03/GTH_Stellu...