PROS1 and infection: While the origins of preexisting cross-reactive antibodies could not be completely explained by previous infections of other coronaviruses, as recent studies revealed that the magnitude of antibody responses to SARS-CoV-2 S protein in the sera of patients with COVID-19 was not related to HCoVs’ S titers45 and immunization with coronaviruses OC43 did not induce significant SARS-CoV-2 S protein cross-reactive antibodies in mice.