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Testing paradox may explain increased observed prevalence of bacterial STIs among MSM on HIV PrEP: A modeling study

Laura Müller1,2, Piklu Mallick1,2, Antonio B Marín-Carballo1,3

  • 1Max Planck Institute for Dynamics and Self-Organization, Göttingen 37077, Germany.

Proceedings of the National Academy of Sciences of the United States of America
|October 29, 2025
PubMed

Insights

Pre-exposure prophylaxis (PrEP) may increase STI detection rates, not necessarily transmission, due to enhanced screening. Optimizing screening strategies is crucial for PrEP programs to effectively reduce HIV and STI incidence.

Area of Science:

  • Epidemiology
  • Mathematical Modeling
  • Public Health

Background:

  • HIV pre-exposure prophylaxis (PrEP) implementation coincides with rising bacterial STI rates among men who have sex with men.
  • This trend raises questions about potential unintended consequences of PrEP on STI transmission dynamics.

Purpose of the Study:

  • To develop a dynamical model reconciling conflicting epidemiological observations regarding PrEP and STI rates.
  • To investigate the impact of risk-mediated behavior, reduced condom use, and asymptomatic screening on simultaneous HIV and STI transmission.

Main Methods:

  • Developed a minimal dynamical model for co-transmission of HIV and chlamydia.
  • Integrated mechanisms: risk-mediated behavior, reduced condom use in PrEP users, and PrEP-associated STI screening.
  • Systematically mapped parameter space including PrEP uptake, screening frequency, and risk perception.

Main Results:

  • Identified a "testing paradox" where true STI prevalence may decline while observed detection rates rise.
  • Demonstrated that increased PrEP uptake amplifies screening intensity, inflating detection.
  • Found conditions under which this paradox arises based on PrEP uptake, screening, and risk perception.

Conclusions:

  • Reconciled conflicting epidemiological evidence on PrEP and STI dynamics.
  • Highlighted the critical role of asymptomatic screening strategies in determining the net effect of PrEP on STI dynamics.
  • Emphasized aligning screening and treatment policies to maximize benefits (reduced HIV/STI incidence) and minimize risks (e.g., antimicrobial resistance).
Keywords:
HIV PrEPMSMSTIasymptomatic screeningpublic health

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