Time course of systemic NCP antigenemia in COVID-19 patients
Next, we compared NCP serum concentrations between non-ICU and ICU patients. During the first week, NCP antigen was detected in 36/43 samples from ICU patients (83.7%) and 23/25 samples from non-ICU patients (92.0%, Fig. 1A) with similar peak values (Fig. 1B). After day 8 post COVID-19 PCR diagnosis, non-ICU patients had invariably cleared NCP (14/14, 100%). In contrast, NCP was still elevated in 26/57 (45.6%) samples from ICU patients obtained during the late phase of illness (>8 days, Fig. 1B). Mean NCP antigen concentrations remained significantly higher in ICU compared to non-ICU patients after the 8th day of illness (Fig. 1B). Importantly, the Ct values in swab PCRs from ICU patients above vs. below the NCP cutoff did not differ significantly (Fig. S3A-C), suggesting that NCP antigenemia is independent of mucosal clearance. Thus, while high NCP antigenemia was a phenomenon in all hospitalized COVID-19 patients during the first week of illness, prolonged systemic circulation of NCP beyond the first week was detected only in ICU patients.