Pilots are encouraged to be aware of the signs of deep venous thrombosis and clotting issues and take preventative measures such as compression stockings and stretching of their legs during long flights.
Medical News Today published a study on June 15th, 2021 that showed an increased risk of blood clotting and low platelets in AstraZeneca COVID-19 vaccine recipients. Some scientists hypothesize that since the ethylenediaminetetraacetic acid (EDTA) in the shot causes a full body reaction, once the vaccine comes into contact with platelets inside the human body, the vaccine activates those platelets, causing them to change shape and transmit chemical signals to the immune system. Those platelets send out platelet factor 4 (PF4), which regulates blood clotting.
However, in some people, after some undetermined amount of time, at random, PF4 latches onto the vaccine, and large "complexes" form. Since those complexes are "unknown," the human body interprets those clusters as threats. Thus, immune cells in the body mistakenly attack PF4's, prohibiting them from preventing the problematic clots seen in some COVID-19 vaccine recipients.