Anti-PF4/polyanion IgG in relation to the time point of vaccination (day 0) with BNT162b2 and ChAdOx1 nCoV-19. An OD of >0.5 units was considered positive (gray shaded area: reference range OD <0.5). SeCo substudy: (A-B) Sera of 111 individuals who were vaccinated twice with BNT162b2 (A); 123 individuals vaccinated once with ChAdOx1 nCoV-19 (B). (C-D) Seropositive subjects with available baseline samples vaccinated with BNT162b2 (C) and vaccinated with ChAdOx1 nCoV-19 (D). Baseline samples were taken at a median of 261 days before BNT162b2 and at a median of 169 days before ChAdOx1 nCoV-19 vaccination. (E-F) AICOVI substudy: Sera of 47 participants tested at prevaccination baseline (day 0) and at days 7 and 14 after 2 doses of BNT162b2 (first dose, day 0; second dose, day 28) (E), and 1 dose of ChAdOx1 nCoV-19 (day 0) (F). Subjects with increasing ODs after vaccination are shown by triangles.