- SARS-CoV-2 specific CD4+ T cells can be detected as early as 2-4 days post-symptom onset
- CD4+ T cells also had the strongest association with lessened COVID-19 disease severity
- the absence of SARS-CoV-2 specific CD4+ T cells was associated with severe or fatal COVID-19

State University of New York at Stony Brook

Any viral infection leads to a decrease in lymphocyte levels but the change varies from virus to virus. In the case of COVID-19, it has been found out that CD8+ cells (Cytotoxic T cells) play an important role in predicting the severity of the infection and therefore are crucial in designing a proper treatment method. It has been observed that the levels of CD4+ and CD8+ cells decrease drastically in severe cases but not so much in milder ones. Even though a change in CD4+ to CD8+ ratio is not seen. It was observed that the CD4+ and CD8+ levels start rising as the body responds to treatment. 

Effect of COVID-19 on T-cell levels in infected patients

- CD4+ T cell responses are more responsible for controlling SARS-CoV-2 infection than CD8+ T cells
- induction of antibodies also depend on spike-specific CD4+ T cells
- in a study examining CD4+ T cell responses to all SARS-CoV-2 proteins, CD4+ T cell responses were detectable for all but three of the smallest proteins
- S spike protein, M, and nucleocapsid were the main targets of CD4+ T cells
- virus specific CD4+ T cells differentiate into Th1 cells and T follicular helper cells (Tfh)

CD4+ T cell response in SARS-CoV-2 infection

Sette, A., & Crotty, S. (2021). Adaptive immunity to SARS-CoV-2 and COVID-19. Cell, S0092-8674(21)00007-6. Advance online publication. https://doi.org/10.1016/j.cell.2021.01.007

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