NCP antigenemia and SARS-CoV-2-specific antibody responses
In sera of hospitalized COVID-19 patients without detectable antibodies, NCP antigen was detected in high concentrations >100 pg/mL (Fig. S1A,B). As expected, a subset of patients with detectable antibodies against S1 or NCP had NCP antigen levels <2.97 pg/mL, consistent with successful clearance of the infection. We assumed that the appearance of NCP-specific IgG would show a strong association with the reduction of NCP antigenemia. Instead, we found that only 33.0% of anti-NCP-positive, but 40.6% of anti-S1-positive sera had an NCP antigen concentration below the cutoff. Simultaneous detection of antibodies and NCP antigen was also found (29.3% of S1 IgG-positive sera, 38.3% of NCP IgG-positive sera, Fig. S1A,B). Accordingly, NCP antigen was lower in anti-S1-positive compared to anti-NCP-positive sera (Fig. S1C), suggesting that S1-specific IgG is more relevant than NCP-specific IgG to reduce NCP antigenemia.