A new coronavirus infection caused by the SARS-CoV-2 virus, which appeared in December 2019, has claimed the lives of 2.5 million people in almost a year. The high contagiousness of this virus has led to its wide and rapid spread around the world. As of February 2021, the total number of cases is 111 million people; more than 4 million cases of SARS-CoV-2 infection have been registered in the Russian Federation. To successfully combat the emerging pandemic, it is necessary to quickly diagnose the disease at an early stage, which will prevent the further spread of this virus and prescribe the necessary treatment on time. The aim of the work was to evaluate the use of the SARS-CoV-2 nucleocapsid antigen (N-Ag) and respective antibodies as diagnostic markers in pneumonia patients. The study was conducted at the height of COVID-19 pandemic in Moscow, Russia. It included 425 emergency patients with clinical signs of COVID-19 pneumonia, of which 280 (66%) were positive for either serum N-Ag and/or its respective antibodies. We demonstrate the total prevalence of N-Ag seroconversion in SARS-CoV-2associated pneumonia patients within 3–5 days after hospital admission. The results indicate high feasibility of SARS-CoV-2 serodiagnostics in emergency patients.