PROS1 and Arthritis: We initiated our work by conducting a bibliographic search in the Medline database (1946 to 1 July 2021) using the following keywords: VZV, chickenpox, varicella, varicella zoster virus, arthritis, “idiopathic purpura fulminans,” “acquired protein S deficiency,” and “anti-protein S antibodies.” The search was limited to publications in English, and the references in the articles were reviewed for all cases of varicella arthritis and purpura fulminans.