It is well known that covid-19 is prone to develop severe diseases in the elderly and underlying diseases (such as diabetes, cerebrovascular diseases and hypertension etc.).
There are various discussions about the difference between the elderly and the young, such as that young people have stronger innate immunity, and in the elderly, the balance between innate and acquired immunity is broken.
The following group believes that why COVID-19 becomes severe in the elderly would be related to the presence or absence of T-cell cross-reactions specific to conventional human coronaviruses (strains such as HCoVs: NL63, OC43).
https://www.nature.com/articles/s41598-020-78506-9
From this study, it was confirmed that T-cell immunity induced by human α-HCoV (NL63) and β-HCoV (OC43) was present in young adults, but virtually not in the elderly, and the frequency of cross-reactive T-cells directed to novel coronavirus (SARS-CoV-2) was minimal in most elderly. Indeed, it is likely to be related to the severity of the elderly.
Difference in IFN-γ response,
<10 (black), 10–30= (dark gray), 30–100 (green), 100–200 (light gray), >200 (Orange) SFU/10^6 PBMC.