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Background: In view of the scientific gap in knowledge of the involvement of the B-cell compartment and clinical prognostic in SARS-CoV-2 infection, this work aims to evaluate the B-cell subsets and the presence of specific IgM and IgG, as well as neutralizing antibodies against SARS-CoV-2, in unvaccinated patients diagnosed with COVID-19. Methods: This study included 133 patients with COVID-19. Cellular components were assessed by flow cytometry, and immunoglobulin levels and reactivity were measured by indirect enzyme-linked immunosorbent assay. Results: Our results showed no changes in less differentiated B cells. However, non-switched memory B cells (NS-MBCs) and class-switched memory B cells (CS-MBCs) were reduced in the patients with moderate disease. Also, plasmablasts and double-negative (DN) or "atypical" memory B cells were increased in groups of patients with moderate to critical conditions. In addition, the production of IgM, IgG, and neutralizing antibodies against SARS-CoV-2 demonstrated a positive correlation between the positivity of antibodies against SARS-CoV-2 and disease severity. Besides being related to the development of a more severe course of the disease, the increase in DN B-cell count also contributed to a poorer disease outcome in patients with a higher percentage of these cells. On the other hand, we observed an increase in the absolute number of CS-MBCs in patients with greater chances of survival. Conclusions: This study demonstrates that the B-cell compartment may contribute to the development of clinical symptoms of COVID-19, with changes in B-cell subset counts linked to disease course and patient prognosis. [ABSTRACT FROM AUTHOR]

Coronavirus disease 2019 (COVID‐19) is a respiratory tract infection caused by the new severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2). An adequate T cell response is essential not only for fighting disease but also for the creation of immune memory. Thus, the present study aims to evaluate the T cells of patients with moderate, severe and critical COVID‐19 not only at the time of illness but also 2 months after diagnosis to observe whether changes in this compartment persist. In this study, 166 COVID‐19 patients were stratified into moderate/severe and critical disease categories. The maturation and activation of T cells were evaluated through flow cytometry. In addition, Treg cells were analysed. Until 15 days after diagnosis, patients presented a reduction in absolute and relative T lymphocyte counts. After 2 months, in moderate/severe patients, the counts returned to a similar level as that of the control group. In convalescent patients who had a critical illness, absolute T lymphocyte values increased considerably. Patients with active disease did not show differentiation of T cells. Nonetheless, after 2 months, patients with critical COVID‐19 showed a significant increase in CD4+ EMRA (CD45RA+ effector memory) T lymphocytes. Furthermore, COVID‐19 patients showed delayed T cell activation and reduced CD8+ suppressor T cells even 2 months after diagnosis. A reduction in CD4+ Treg cells was also observed, and their numbers returned to a similar level as that of healthy controls in convalescent patients. The results demonstrate that COVID‐19 patients have a delayed activation and differentiation of T cells. In addition, these patients have a great reduction of T cells with a suppressor phenotype. [ABSTRACT FROM AUTHOR]