Introduction We have previously described Western Australia as a “hotspot” for HIV-1 subtype diversity in Australia. This investigation characterises this further by studying phylogenetic transmission networks in relation to HIV clinical parameters, from 2000–2014.

Methods Baseline clinical data and HIV-1 pol sequences were assessed over 4 notification eras for 1021 patients. Phylogenetic tree construction (MEGA V6) was utilised to identify transmission networks, using clustering criteria of bootstrap ≥98 and genetic distance ≤1.5%.

Results The proportion of non-B-subtype HIV-1 has remained stable from 2008–2014 (35% of males (subtype CRF01_AE > C); 80% of females (subtype C > CRF01_AE). Non-B-subtype HIV-1 was associated with reduced baseline CD4 count (p = 0.005) after adjusting for effects of baseline viral load and age (p < 0.001).

More and larger transmission clusters were identified among the B-subtype group (p < 0.05), with one cluster of 53 individuals evolving from 2008, characterised by higher baseline CD4 count (p = 0.001) and viral load (p = 0.01) than ungrouped patients. This cluster has expanded in 2014 (12 new cases) despite high proportions of early diagnoses (25% with acute HIV-1 serology) and treatment uptake (76% with HIV VL <40cpm by 2014).

Conclusion This 14-year study highlights several challenges in HIV-1 prevention, including delayed diagnosis among cases of non-B-subtype HIV-1, namely migrants and overseas travellers. We have also identified a substantial increase in baseline viral load over time, with higher viral load levels within a large transmission cluster that continues to expand despite frequent early diagnosis and high treatment uptake. These results can inform strategies to end HIV transmission within Australia.

Disclosure of interest statement None to disclose.