Whilst patients who reported thrombosis due to COVID-19 vaccines probably had no history of heparin therapy, proinflammatory cytokines such as interleukin-6 (IL-6), interleukin-1 (IL-1), and tumor necrosis factor-α (TNF-α) may be responsible for the spontaneous formation of platelet-activating anti-PF4/heparin antibodies similar to those formed in heparin-induced thrombocytopenia26. Here, PF4 is linked to deep vein thrombosis.