The aim of this cross-European study involving 5 laboratories in Germany, France, and Italy was to evaluate the diagnostic performance of the COVID19SEROSpeed IgM/IgG rapid test (abbreviated “BioS” here) for the detection of antibodies against SARS-CoV-2 in comparison to those of well-established commercial ELISAs and CLIAs and to analyze the kinetics of antibody response with reference to onset of symptoms in severe and nonsevere COVID-19. This evidence concerns the gene CD40LG and COVID-19.