In type 1 immunity, pathogen clearance is mediated through effector cells including group 1 innate lymphocytes (ILC1), natural killer (NK) cells, cytotoxic T lymphocytes, and T helper 1 (TH1) cells


It is clear from this and other studies that the immune response in hospitalized patients with severe COVID-19 is characterized by lymphopenia and the expression of molecules associated with ongoing inflammation8, whereas these same molecules are expressed at a lower level in people with mild or moderate disease. Differences in immune responses between the different categories of disease severity are even more evident when people with very mild or subclinical disease are included in the analyses.

A key next step will be to analyse samples from people with extremely early signs of COVID-19, and to compare longitudinal data in those who do and those who don’t require hospitalization. Some people who develop severe disease seem to have a suboptimal immune response initially, which might allow uncontrolled viral replication. Such high replication might, in turn, contribute to severe disease.

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related { Efforts are ongoing to find which human or viral factors underpin whether a person with COVID-19 will develop severe symptoms. Clinical evidence linked to two viral lineages now provides key insights into this enigma. | Nature }

quote { Longitudinal analyses reveal immunological misfiring in severe COVID-19 }