Furthermore, 9 potential cross-reactive sera were included, i.e. IgM positivity against Cytomegalovirus (CMV) (n = 2), Epstein-Barr virus (EBV) (n = 2) or Toxoplasma gondii (Toxo) (n = 1); samples positive for rheumafactor (RF) (n = 1) or anti-nuclear factor (ANF) (n = 1)) and sera from patients positive for non-SARS-CoV-2 endemic coronaviruses (OC43 and NL63) (n = 2).