Antibody responses in NCP-positive ICU patients
To understand the possible defect in ICU patients that have prolonged NCP antigenemia (after 8 days of COVID-19), ICU patients were grouped into those with NCP levels above and those below the cutoff, and the S1- and NCP-specific antibody responses were analyzed. ICU patients without detectable antigenemia had antibodies against S1 (Fig. 2A) and NCP (Fig. 2B) with ratios similar to non-ICU patients, suggesting normal antibody responses. In contrast, ICU patients with antigenemia after day 8 of COVID-19 had significantly lower S1-specific antibodies compared to ICU patients without antigenemia. This difference was restricted to S1-specific antibodies, while no difference was found for IgG against NCP (Fig. 2B). Thus, the prolonged NCP antigenemia in COVID-19 patients on ICU was associated with decreased antibody responses against S1, but not NCP.