ABSTRACT
If age boundaries are arbitrarily or roughly defined, age-related analyses can result in questionable findings. Here, we aimed to delineate the uniquely age-dependent immune features of coronavirus disease 2019 (COVID-19) in a retrospective study of 447 patients, stratified according to age distributions of COVID-19 morbidity statistics into well-defined age-cohorts ( 2-25y, 26-38y, 39-57y, 58-68y, and 69- 79y). Agedependent susceptibilities and severities of the disease were observed in COVID-19 patients. A comparison of the lymphocyte counts among the five age- groups indicated that severe acute respiratory syndrome coronavirus 2 (SARS- CoV-2) infection led to age-dependent lymphopenia. Among the lymphocyte subsets, the CD8(+) T cell count alone was significantly and age-dependently decreased (520, 385, 320, 172, and 139 n/ mu l in the five age-groups, respectively). In contrast, the CD4(+) T cell, B cell, and natural killer cell counts did not differ among age-cohorts. Age and CD8(+) T cell counts ( r=.0.435, p<0.0001) were negatively correlated in COVID- 19 patients. Moreover, SARS-CoV-2 infection age-dependently increased the plasma C-reactive protein concentrations (2.0, 5.0, 9.0, 11.6, and 36.1 mg/L in the five age-groups, respectively). These findings can be used to elucidate the role of CD8+ T cells in age-related pathogenesis and to help develop therapeutic strategies for COVID-19.