PF4 and COVID-19: In this study, we investigated IgG anti-PF4/H levels in several populations including i) a cohort of control patients, from before 2010; ii) 2 French cohorts of control individuals included in 2020 and 2021 before and after COVID-19 vaccination at various sampling times, and using different COVID-19 vaccines; iii) a cohort of patients with SID before and after COVID-19 vaccination [25]; and iv) a cohort of consecutive patients referred to our center and presenting with thrombotic events within 30 days after COVID-19 vaccination (Supplementary Table S8).