Abstract
In 2020, as the severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2) pandemic spread rapidly throughout the world, scientists worked relentlessly to develop and test the safety and effectiveness of potential vaccines. Usually, the vaccine development process involves years of investigation and testing prior to gaining approval for use in practice. A pathogenic PF4-dependent syndrome, unrelated to the use of heparin therapy, may be manifested following the administration of viral vector vaccines. It leads to severe clot formation at unusual sites approximately in 1 out of 110.000 vaccinated persons. This side effect, although rare, represents a newly devastating clotting phenomenon manifested in otherwise healthy young adults, who are often female. An in-depth description of the specific biological mechanisms implicated in the syndrome is here summarized.