This is the first study to describe associations between IFNL polymorphisms and immunological outcomes in ART-treated PLWH, individuals with undetectable viral loads on ART and homozygous for rs368234815 [ΔG/ΔG] displayed an increased likelihood of normalisation of the CD4+:CD8+ T-cell ratio, as well as higher proportions of naïve CD8+ T-cells and lower CD4+ effector memory cells in those with HIV mono-infection. Here, CD8A is linked to infection.