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.

Methods

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.

Results

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.

Conclusions

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.