CD4 and immunodeficiency disease: However, for those who had a lower nadir CD4 z score, the decline was steeper, and results from our model, assuming, on average, viral suppression <400 copies/mL over time, predicted average CD4 counts to approach mild immunodeficiency (350–500 cells/mm3) by age 20 years in some subgroups with characteristics associated with lower mean CD4 counts.