When infected with the new coronavirus (SARS-CoV-2) and the severity of COVID-19 progresses, the following changes can be seen in various immune cells
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7591707/
Compared to healthy people,
Leukocytes and neutrophils increase significantly,
Conversely, lymphocytes, phosphates, T cells, and NK cells decrease.
In addition, interesting changes appear in a subset of T cells and NK cells,
the ratio of CD4+/CD8+ increases significantly,
PD-1 NK cells, CD244 NK cells, PD-1 CD4+ T cells, PD-1 CD8+ T cells, CD244 CD8+ T cells increase,
Conversely, CD27 NK cells and CD27 CD8+ T cells are reduced.
Thus, changes in immune cells and their subsets would be related to the mechanism of severity of COVID-19, and are clinically useful for screening of critical illness.