AstraZeneca vaccine is an adenovirus vaccine, which has a double-stranded DNA as its genetic material. The adenovirus is used as a vector for the delivery of the vaccine, but it cannot grow by itself, so the original adenovirus is mutated and its replicating part of the DNA is replaced with the coronavirus spike gene.

After the mass vaccination of AstraZeneca vaccine, some people reported blood clotting, which is an unusual side effect. The blood clotting happened in unusual parts of the body, such as cerebral thrombosis, and occurred in people under the age of 50.

The condition caused by the AstraZeneca vaccine is similar to heparin-induced thrombotic thrombocytopenia (HITT), which is a rare condition that occurs when heparin is used as an anticoagulant. The vaccine-induced condition is called vaccine-induced immune thrombotic thrombocytopenia (VITT).

There are several hypotheses for the cause of VITT, including the possibility that the adenovirus itself is binding to PF4, or that the DNA of the vaccine is negatively charged and binding to PF4. Another hypothesis is that the spike protein produced by the vaccine is binding to PF4 and causing the formation of antibodies.

The treatment for VITT involves the use of anticoagulants that are not heparin-based, as well as the injection of high amounts of immunoglobulin to block the antibodies against PF4. It is also important to check for blood leakage before injecting the vaccine to prevent the vaccine from entering the bloodstream.