BACKGROUND: Gay and bisexual men (GBM) with undiagnosed HIV contribute disproportionately to HIV transmission in Australia. METHODS: In 2014 and 2018 we recruited men at gay venues and events in Sydney. Participants self-completed surveys and provided oral fluid samples for HIV testing. We calculated the prevalence of HIV and undiagnosed infection, and assessed changes in behaviour, HIV testing, and the use of pre-exposure prophylaxis (PrEP). We weighted the samples to adjust for differences in where participants were recruited between rounds. Two-sample tests of proportion were used to compare prevalence estimates and chi-square tests to assess differences between the samples. RESULTS: In 2014, 944 men were recruited and 890 men were recruited in 2018. In 2014, the weighted estimate of HIV prevalence was 6.1% (95%CI 4.6-7.6), of which 13.8% (95%CI 5.0-22.7) was undiagnosed. In 2018, weighted HIV prevalence was 6.4% (95%CI 4.8-8.0), of which 5.3% (95%CI 0.5-11.1) was undiagnosed. Between 2014 and 2018 among all participants, men reporting at least 10 recent casual partners increased from 22.3% to 27.7% (p=0.008), condomless anal intercourse with casual partners in the previous six months increased from 23.9% to 37.3% (p<.001) and STI diagnoses in the previous year increased from 14.4% to 27.5% (p<.001). HIV testing and the use of PrEP in the previous six months increased from 49.6% to 56.3% (p=0.004) and 2.0% to 21.0% (p<.001), respectively. CONCLUSION: Repeated, bio-behavioural surveillance suggests the prevalence of undiagnosed HIV remains low in Sydney, despite GBM reporting more casual sex partners, condomless sex and STIs.
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