Difference in Lymphocyte Subset Counts According to Disease Severity and SARS-CoV-2 Variants of Hospitalized COVID-19 Patients in Korea.

Background: The studies about the characteristics of immune systems and lymphocyte subset counts regarding coronavirus disease 2019 (COVID-19) have been conducted in a number of ways, however studies on the difference of lymphocyte subset counts according to each severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant are rare. As each country has a different quarantine system, it is necessary to investigate the characteristics in places such as South Korea where the vaccination rate is high and a thorough national quarantine system is being operated. The aim of this study was to investigate the difference in peripheral blood lymphocyte subset counts in COVID-19 patients in South Korea according to their disease severity and SARS-CoV-2 variants.

Methods: A total of 148 hospitalized COVID-19 patients enrolled in this study at Eunpyeong St. Mary's Hospital, College of Medicine, The Catholic University of Korea. SARS-CoV-2 infection was confirmed using real-time quantitative polymerase chain reaction and COVID-19 severity was divided into three groups (mild, moderate, and severe) at the time of admission, according to the treatment types of oxygen support. Laboratory tests were performed for evaluation of inflammatory status. The peripheral lymphocyte subset counts were analyzed using flow cytometry at the time of admission in all 148 patients and after treatment in 65 patients with moderate or severe severity. We analyzed the changes of peripheral lymphocyte subset counts according to three COVID-19 severity groups and three SARS-CoV-2 variants (wild and alpha, delta and omicron variants).

Results: At admission, total T, CD4+ T, CD8+ T, natural killer (NK), and NKT cell counts decreased, but B cells did not change significantly depending on severity. There were markedly decreased in total T, CD4+ T, CD8+ T, and NK cells in the delta and omicron variants and not in the wild and alpha variant. After treatment in patients with moderate or severe severity, most lymphocyte subset counts were increased except the NK cells. Lower total lymphocyte counts, especially total T lymphocyte counts were associated with a higher occurrence of severe COVID-19.

Conclusion: The peripheral lymphocyte subset counts decreased according to disease severity and the types of SARS-CoV-2 variants in hospitalized COVID-19 patients. Despite the efficient operation of the national quarantine system and high vaccination rates in South Korea, it suggests that viral mutations evolve in a direction that further weakens the immune system. This is the only study on the difference in peripheral lymphocyte subset counts according to COVID-19 severity and SARS-CoV-2 variants conducted in South Korea, and therefore our data could be used as an important reference for another pandemic situation in the future.