Introduction
Circulation of viral antigen in blood is a mainstay of diagnostic testing in systemic viral infections, including HBV, HCV or HIV. However, in respiratory infections such as SARS-CoV-2, antigenemia in serum is harder to interpret [1, 2]. Systemic presence of nucleocapsid (NCP) as well as spike (S) protein has been described [3]. A high level of serum NCP antigen was consistently found to be associated with serious illness [4-7]. It was suggested that the serum NCP antigen level may serve as an early biomarker for predicting the maximum disease grade of severe COVID-19, using NCP antigenemia as a tool for allocation of ICU options on admission [8]. However, some patients progress to more severe disease not before the second week of Covid-19; for those patients, no biomarker has been established yet. Here, we demonstrate a delay of NCP antigen clearance in patients who required intensive care during hospitalization.