Nevertheless, we suggest that the SARS-CoV-2 S protein was specific for COVID-19 in our urine-based ELISA, since we observed only one false positive for all three proteins from 31 negative individuals using the rSARS-CoV-2 Euk1-S1, Prok1-S1 and Prok2-S1 proteins. Here, PROK1 is linked to COVID-19.