Abstract: A method diagnoses a SARS-CoV-2 infection and includes detecting the presence or absence of an antibody to SEQ ID NO: 1, preferably IgA class antibody, in a sample from a subject. A method differentially diagnoses a coronavirus infection. An antibody to SEQ ID NO: 1, preferably IgA class antibody, is used for diagnosing a SARS-CoV-2 infection or for the differential diagnosis of a coronavirus infection, preferably for distinguishing between a SARS-CoV-2, MERS and NL63, 229E, OC43 and HKU1 infection. A kit includes a polypeptide comprising SEQ ID NO: 1 or a variant thereof, preferably coated to a diagnostically useful carrier and one or more of the following reagents: an antibody to SEQ ID NO: 1, a washing buffer, a means for detecting the presence of an antibody, preferably IgA class antibody, preferably a secondary antibody binding specifically to IgA class antibodies, preferably comprising a detectable label, and a dilution buffer.