PROS1 and respiratory tract infectious disorder: The two-step ELISA protocol is based on published protocols (Stadlbauer et al., 2020), with the seropositive cut-off established using a panel of pre-pandemic sera that showed bacterial pneumonia and other common respiratory infections such as rhinovirus, influenza and respiratory syncytial virus did not cross-react with the SARS-CoV-2 RBD and S proteins.