da Rocha et al.  stressed their interest in the large data base results we presented on HIV-infected patients starting a treatment in the International epidemiological Database to Evaluate AIDS (IeDEA) West Africa Collaboration: the older they are, the poorer their CD4 response to antiretroviral treatment (ART) . They pinpointed two factors that may play a role in the type and level of CD4 response: tuberculosis infection and duration of HIV infection. Indeed, tuberculosis, like other opportunistic infections [3,4], impairs the immune response. The duration of HIV infection has also been reported as an important determinant of the CD4 response to ART [5,6]. When looking at the age effect on the CD4 response, the key point is whether these latter factors play the role of confounding factors or effect modifiers in this association. Hence, with the association between age and tuberculosis being inconsistently described , it is not granted that the measure of the age effect without adjustment for tuberculosis should be biased. Conversely, the duration of HIV infection may indeed confound or modify the effect of age , but the information of time of seroconversion is not available in this large data set collected under routine circumstances for more than a dozen of clinics throughout West Africa.
Rocha et al.  were also concerned by the high percentage of losses for the second point of CD4 measurement’. This underlines the general issue of patients being lost to follow-up in many Africa clinics as we recently highlighted within the IeDEA Collaboration , and this phenomenon could impair estimates because of informative censoring. Innovative approaches have been proposed to chase additional information on these patients [10,11] but were not systematically used in our network. Several sensitivity analyses have been done in the present study to check the robustness of our estimates and were favourable. Also, one should keep in mind that excluding patients with only one measurement may lead to a larger bias than including them using relevant statistical methods, as explained in more detail elsewhere .
In conclusion, our published results should certainly be confirmed in other studies throughout sub-Saharan Africa, with a longer follow-up and taking into account as many determinants as possible of CD4 response. Large networks such as the IeDEA Collaboration are best suited for addressing this type of question (http://www.iedea.org/).
Conflicts of interest
There are no conflicts of interest.
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