The Clinical and Laboratory Landscape of COVID-19 During the Initial Period of the Pandemic and at the Beginning of the Omicron Era

Introduction: Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus has undergone significant mutations since its emergence, resulting in a variety of antigenic variants. One of these variants is Omicron. Methods: In this study, the blood samples from 98 patients with acute coronavirus disease-19 (COVID-19) who were hospitalized during the initial SARS-CoV2 wave and the onset of Omicron infection in 2021 were analyzed. We used high-resolution melting (HRM) of PCR products to analyze RNA extracted from two clinical samples collected in July and November of 2021 from patients infected with the SARS-COV-2 virus. Results: HRM-analysis detected a characteristic deletion in the N-protein RNA of the virus isolated from November 2021 that is associated with the Omicron variant. Elevated levels of C-reactive protein (CRP), neutrophil-lymphocyte ratio (NLR), and interleukin-6 (IL-6) were observed in patients during both initial wave of COVID-19 and 2021 hospitalization. Additionally, complement levels were more frequently detected at the start of hospitalization during the second wave. IgG and IgM antibodies to SARS-CoV-2 were detected more often at the beginning of hospitalization during the second wave of COVID-19. During both outbreaks, an increase in hemagglutinin-inhibiting (HI) antibodies against Influenza A and B was observed in paired blood specimens from moderate to severe COVID-19 patients. Conclusions: Patients admitted during both waves of COVID-19 showed a significant rise in several inflammatory markers, suggesting that Omicron triggers significant inflammatory responses. The rapid formation of IgM and IgG in Omicron patients may indicate a faster immune response due to memory B cell formation after previous infections or vaccinations. Seasonal flu may negatively impact the clinical course of coronavirus infections. Further research is needed to determine if clinical presentation and laboratory parameters change in response to variations in the SARS-CoV virus in breakthrough cases and in patients with post-COVID syndrome.