Reducing STI Burden in MSM with Doxy-PEP: Evidence from Individual-Based Modelling in Australia

Objectives Syphilis, gonorrhoea, and chlamydia are commonly diagnosed sexually transmitted infections (STIs) among men who have sex with men (MSM). Doxycycline post-exposure prophylaxis (Doxy-PEP) could be an effective public health intervention to reduce STI incidence. In this modelling study, we evaluate potential implementation strategies for Doxy-PEP roll-out to Australian MSM to inform clinical guidelines.

An individual-based mathematical model was developed to simulate the transmission of syphilis, gonorrhoea, and chlamydia within an urban MSM population in Australia. Individuals in the model form and dissolve regular and casual partnerships at rates based on publicly available sexual behaviour data, with infections transmitted through sexual contact within these partnerships. The impact on STI incidence over five- and ten-year periods was evaluated under different Doxy-PEP eligibility criteria, including HIV infection status and/or STI diagnosis history.

Offering Doxy-PEP to individuals living with HIV and to current HIV pre-exposure prophylaxis (PrEP) users, or those with more than one positive STI diagnosis in the previous 12 months is estimated to reduce syphilis incidence by over 50% within 5 years. The incidence of gonorrhoea and chlamydia is predicted to decrease by over 40%, but the reduction in gonorrhoea incidence diminishes to less than 10% if Doxy-PEP efficacy against gonorrhoea declines over time due to increasing antimicrobial resistance.

Doxy-PEP could significantly reduce STI incidence, with the greatest impact observed for syphilis. However, the impact on gonorrhoea incidence may not be sustainable if Doxy-PEP efficacy wanes. Ongoing monitoring of Doxy-PEP efficacy and adherence is critical for reductions in STI incidence to be sustained. Key messages What is already known on this topic Clinical trials have shown that doxycycline post-exposure prophylaxis (Doxy-PEP) can reduce the risk of acquiring bacterial STIs such as syphilis, gonorrhoea, and chlamydia. Given the potential resurgence of STIs among priority populations in Australia, the NSW Ministry of Health has considered including Doxy-PEP as part of its response to STIs. This study uses mathematical modelling to assess the potential epidemiological impact of prescribing Doxy-PEP to men who have sex with men (MSM) following a STI diagnosis. What this study adds The modelling suggests that introducing Doxy-PEP based on HIV status/HIV PrEP usage, or STI diagnosis could reduce syphilis and chlamydia incidence by over 40% within five years. A similar reduction in gonorrhoea incidence may also be possible; however, the impact could be limited to just 8% if antimicrobial resistance to doxycycline is already present or emerges because of the intervention. How this study might affect research, practice or policy Implementing Doxy-PEP alongside STI testing could reduce syphilis incidence among MSM in Australia. Similar reductions in other STIs may also be achievable; however, long-term effectiveness may be threatened by the emergence of doxycycline resistance associated with widespread Doxy-PEP use. To sustain these benefits, high uptake and adherence to Doxy-PEP, along with robust monitoring for antimicrobial resistance, will be essential. Competing Interest Statement The authors have declared no competing interest. Funding Statement This project was supported by the BBV & STI Research, Intervention and Strategic Evaluation (BRISE) program, funded by the NSW Ministry of Health. The Kirby Institute is funded by the Australian Government Department of Health and Aged Care. The views expressed in this publication do not necessarily represent the position of the Australian Government This research was produced in whole or part by UNSW Sydney researchers and is subject to the UNSW intellectual property policy. For the purposes of Open Access, the author has applied a Creative Commons Attribution CC-BY public copyright licence to any Author Accepted Manuscript (AAM) version arising from this submission. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: The study used ONLY openly available human data that were originally located at: https://www.kirby.unsw.edu.au/report-type/annual-surveillance-reports I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability The modelling code is available online thru GitHub All other data produced in the present study are available upon reasonable request to the authors