This is consistent with previous studies that showed that HIV co-infection with syphilis is associated with systemic inflammation characterised by elevations in CRP levels.32 The lower CD4 cell counts in participants with co-infections could be the consequence of immune activation.33 We found lower plasma iron levels at baseline in participants with co-infections, likely due to the pro-inflammatory state, which results in the liver releasing hepcidin which in turn signals the ferroportin channels to block the efflux of iron into circulation, resulting in reduced plasma iron levels.34 Here, CRP is linked to syphilis.