Syphilis infection may potentiate transmission of HIV. We sought to determine the extent to which HIV acquisition was associated with syphilis infection within an HIV pre-exposure prophylaxis trial (iPrEx) and whether Emtricitabine/Tenofovir (FTC/TDF) modified that association.
The Pre-exposure Prophylaxis Initiative (iPrEx) study randomly assigned 2499 HIV-seronegative men and transgender women who have sex with men (MSM) to receive oral daily TDF coformulated with emtricitabine (FTC/TDF) or placebo. Syphilis prevalence at screening and incidence during follow-up were measured. Hazard ratios for the effect of incident syphilis on HIV acquisition were calculated. The effect of FTC/TDF on incident syphilis and HIV acquisition was assessed.
Of 2499 individuals, there were 360 (14.4%) with positive RPR at screening; 333 (92.5%) had a positive confirmatory test, which did not differ between the arms (FTC/TDF vs placebo, p=0.81). The overall syphilis incidence during the trial was 7.3 cases per 100 person years. There was no difference in syphilis incidence between the study arms (FTC/TDF: 7.8 cases/per-year vs. placebo: 6.8, p=0.304). HIV incidence varied by incident syphilis (no syphilis: 2.8 vs incident syphilis: 8.0 cases per 100 person-years) reflecting a hazard ratio (HR) of 2.6, 95% confidence interval (CI) 1.6 to 4.4, p<0.001. There was no evidence for interaction between randomization to the FTC/TDF arm and incident syphilis on HIV incidence.
In HIV-seronegative MSM, syphilis infection is associated with HIV acquisition in this pre-exposure prophylaxis (PrEP) trial; a syphilis diagnosis should prompt providers to offer PrEP unless otherwise contraindicated.
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