Enhanced screening and bacterial sexually-transmitted infection diagnoses after HIV pre-exposure prophylaxis initiation

Background: Recipients of HIV pre-exposure prophylaxis (PrEP) experience higher rates of gonorrhea and chlamydia diagnoses than non-recipients. However, it is unclear if these observations reflect a causal relationship between PrEP initiation and acquisition of sexually transmitted infections (i.e., behavioral “risk compensation”) or a diagnostic bias related to PrEP recipients screening more frequently than non-recipients. Methods: We conducted a self-controlled case series study comparing rates of gonorrhea and chlamydia diagnoses after versus before PrEP initiation among commercially-insured US males in the Merative MarketScan® Research Databases (2016–2019). We compared matched incidence rate ratio (IRR) estimates after versus before PrEP initiation to model-based expectations under a null hypothesis of no change in infection risk. We derived null expectations via a mathematical model reflecting changes in screening after PrEP initiation, estimating parameters under a Bayesian framework. Results: Individuals experienced increased rates of gonorrhea (IRR=3.47 [95% confidence interval: 2.57-4.72]) and chlamydia (IRR=3.59 [2.50-5.27]) diagnoses after PrEP initiation, with IRR estimates differing by anatomical site (IRR=1.23 [0.71-2.13] and 5.98 [3.63-10.29] for urogenital and extragenital diagnoses of either infection, respectively). After accounting for increased rates of screening after PrEP initiation, rates of gonorrhea and chlamydia diagnoses only modestly exceeded expectations under the null hypothesis of no change in infection risk (8-14% greater-than-expected rates of gonorrhea and chlamydia diagnoses, respectively). Overall, we estimated that 80-81% of observed increases in rates of gonorrhea and chlamydia diagnoses after PrEP initiation, respectively, were attributable to increases in asymptomatic screening. Conclusions: Our findings suggest higher-frequency asymptomatic screening, rather than behavioral risk compensation, may account for increased rates of gonorrhea and chlamydia diagnoses after PrEP initiation.