Background: Since 2007, rates of new HIV diagnoses in San Francisco (SF) decreased with widespread HIV testing, pooled HIV RNA testing for high-risk seronegatives, increased viral suppression rates, and grass-roots initiatives. Consumer demand for pre-exposure prophylaxis (PrEP) has increased since mid-2013. Local goals for PrEP scale-up have not been established.
Methods: A simple model was developed to forecast HIV transmission with expanded PrEP use. The model considers infectiousness and partnering practices of diagnosed and undiagnosed persons with HIV infection, viral suppression rates, and transmission to uninfected people having low, moderate, or high numbers of partners. Model parameters for SF were derived from surveillance, local research on seroadaptive behaviors, and SF-specific data from cohort studies, including the iPrEx Open Label Extension (OLE). Adherence in OLE was monitored by drug concentrations in dried blood spots and mapped to efficacy using global iPrEx data. The optimistic scenario assumes PrEP uptake will attract and retain people with higher exposure to HIV, as was observed at SF’s OLE site. The realistic scenario assumes incidence rates that are typically observed in SF cohorts that did not include access to PrEP.
Results: In SF, the HIV diagnosis rate is 94% with 67% viral suppression. Among 150 eligible participants in OLE in SF, 64% chose to start PrEP; People starting PrEP were more likely to report non-condom receptive anal intercourse (44% vs 26%; P=0.03). Adherence yielded substantially protective drug concentrations among 96% of users through week 24, falling slightly afterward. If PrEP were used by 6400 people in the optimistic scenario (incidence 1.3 to 4.2/100py), the number of new infections could fall by 50% city-wide; doubling the number on PrEP could reduce new infections to less than 50 per year, a 86% reduction. In the realistic scenario (incidence 0.8 to 2.5/100 py), the city-wide incidence falls by 30% with 6400 people on PrEP; getting to less than 50 cases a year requires that diagnosis rates increase to over 99% with 90% viral suppression, at which point PrEP’s impact on HIV incidence decreases because exposure to untreated HIV infection would be rare.
Conclusions: Demand for PrEP is increasing in SF with high rates of adherence. Widespread use of PrEP could markedly decrease new HIV infections, especially if synergies between PrEP uptake and adherence, HIV exposure, and HIV testing continue during PrEP rollout.