O6.2 - Syphilis testing, incidence, and reinfection among gay and bisexual men with and without HIV in Australia over a decade spanning HIV PrEP implementation
Changes in condom use and serosorting associated with HIV PrEP and treatment as prevention may be driving syphilis transmission in gay and bisexual men (GBM). We describe trends in syphilis testing and incidence among a large, national, retrospective cohort of GBM in Australia over a decade spanning PrEP implementation.
Methods
Data were extracted from 55 sentinel clinics participating in the ACCESS project. GBM with ≥2 visits and ≥2 syphilis tests from Jan2012-Dec2021 were included in testing and incidence analyses, respectively. Annual rates of syphilis testing and diagnosis (primary, secondary, early-latent) per 100 person-years (py) were calculated disaggregated by HIV status and PrEP use (evidence of PrEP prescription; dichotomised as ever-PrEP vs never-PrEP). Poisson regression tested for trends from 2012-2021. Among those diagnosed with syphilis, rate of syphilis reinfection was calculated. Multivariable Cox regression explored risk factors for syphilis between 2016 (post-PrEP availability) to 2019 (pre-COVID).
Results
135,443 GBM (mean age=35.9 years; 7.8% with HIV; 31.0% ever-PrEP-users) had ≥2 visits and 93,918 GBM had ≥2 syphilis tests. There were 653,438 syphilis tests (median days between tests=119) over 610,996 person-years (testing rate=107.0/100py); testing was highest among GBM with HIV (152.2/100py). Between 2012-2021, testing increased from 68.3/100py to 132.7/100py (p< 0.001), mostly driven by increases among ever-PrEP-users[Figure]. There were 13,662 syphilis infections diagnosed in 9,546 (10.2%) GBM over 399,399 person-years (incidence rate=3.4/100py). Syphilis incidence was highest among GBM with HIV (6.7/100py), followed by ever-PrEP-users (3.7/100py) and never-PrEP-users (1.6/100py). Between 2012-2021, syphilis incidence increased among ever-PrEP-users from 1.4/100py to 5.0/100py (p< 0.001), and fluctuated between 5.8/100py and 7.4/100py among GBM with HIV. Rate of syphilis reinfection was 12.5/100py, and similar across groups. Ever previously diagnosed with syphilis (adjusted Hazard Ratio [HR]=2.63, 95%CI=2.45-2.82) was the strongest predictor of syphilis incidence, followed by recent (< 12m) syphilis diagnosis (aHR=2.04, 95%CI=1.89-2.20), living with HIV (aHR=2.07, 95%CI=1.94-2.23) and recently (< 12m) prescribed PrEP (aHR=1.91, 95%CI=1.77-2.06).
Conclusions
Syphilis trends between GBM with HIV and GBM with evidence of PrEP use have been converging over the past decade. Previous syphilis infection was the strongest predictor of subsequent infection, highlighting the importance of including prior diagnosis as an indication for new targeted strategies (e.g. doxy-PEP).
