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.