Autoantibodies targeting PF4 have been implicated in the thrombosis and thrombocytopenia syndrome (TTS) induced in rare cases following vaccination with the ChAdOx1 nCoV-19 (AstraZeneca) or the Ad26.COV2.S (Johnson & Johnson) COVID-19 vaccines (Greinacher et al., 2021; Muir et al., 2021; Scully et al., 2021). Here, PF4 is linked to COVID-19.