The CD4+ T-cell depletion model that was adopted by Lodi et al. and Touloumi et al. [50, 51] can be expressed as\documentclass[12pt]{minimal}\usepackage{amsmath}\usepackage{wasysym}\usepackage{amsfonts}\usepackage{amssymb}\usepackage{amsbsy}\usepackage{mathrsfs}\usepackage{upgreek}\setlength{\oddsidemargin}{-69pt}\begin{document} $$\sqrt{CD4(t)}=a_{i}+(b_{i}\times t)+e_{it},$$ \end{document}CD4(t)=ai+(bi×t)+eit, where t denotes the time from infection to the date of the first CD4+ T-cell count determination. Here, CD4 is linked to infection.