The study by Song et al analyzing memory B cells from convalescent COVID-19 patients supports that elevated antibody levels to HCoV-HKU1 S-protein in COVID-19 donors is consistent with the former, as SARS-CoV-2 activates B cells expressing preexisting HCoV-HKU1 S-specific B cell receptors to secrete the corresponding antibodies [6]. Here, PROS1 is linked to COVID-19.