CD4 and COVID-19: In healthy adult population there is generally a strong correlation between humoral and cellular responses against SARS-CoV-2 after natural infection: 96% of mild COVID-19 patients have positive SARS-CoV-2 S-protein–IgG until 8 to 12 months after infection, and memory CD4+ and CD8+ T-cells up to 12 months after infection (8).