Dear Editors,We read with great interest the recent paper by Rossotti et al. which showed that direct-acting antiviral (DAA) treatment may improve inflammatory markers levels, without significant changes in CD4/ CD8 ratio, over a 12-week follow-up in HIV/hepatitis C virus (HCV) co-infected individuals. 1 The clinical utility of CD4/CD8 ratio monitoring in people living with HIV (PLWH) is increasingly recognized, especially among individuals with controlled infection. 2 Substance (including tobacco, alcohol and cannabis) use, frequently reported by PLWH, is rarely cited as a determinant of CD4/CD8 variations, 2 while it may impact CD4 and CD8 cell counts. 3 We therefore investigated the relationships between substance use and CD4/CD8 ratio in HIV/HCV co-infected people receiving DAA treatment, to test at the same time whether Rossotti et al. findings could be confirmed in this population.We selected HIV/HCV co-infected participants in the French, prospective, observational ANRS CO13 HEPAVIH cohort, 4 who initiated a DAA treatment and had both an undetectable HIV viral load at the time of DAA initiation (baseline) and an undetectable HCV RNA 12 weeks after the DAA treatment end. The participants completed a socio-behavioural questionnaire at baseline and at DAA treatment end, which investigated their use of alcohol, tobacco and cannabis. We used clinical and biological data collected LETTER TO THE EDITOR