Background:  Expanded use of ART may reduce HIV incidence and eventually HIV prevalence at the population level, assuming significant behavioral HIV risk disinhibition does not occur. In San Francisco, where HIV prevalence among MSM is 23% and the majority is aware of their HIV serostatus, we sought to estimate the effects of expanded ART treatment strategies on the San Francisco MSM HIV epidemic.     

Methods:  We used a mathematical model to quantify the effect of 3 expanded ART treatment strategies on the MSM HIV epidemic in San Francisco: 1) ART treatment for all adults currently in care at CD4<500, 2) ART for all adults in care regardless of CD4 count, 3) ART for all adults in care plus community-wide annual HIV testing, comparing these strategies to the current practice of ART for CD4 <350. Modeling was done in R using ordinary differential equations structured as a compartment model with uninfected persons and 4 stages of HIV disease (CD4:<200, 200 to 350, 350 to 500, 500+) and 4 subcategories (undiagnosed HIV, diagnosed HIV untreated, treated partially suppressed, and treated fully suppressed) and high and low behavioral risk populations. Model input parameters, population sizes, and progression rates were taken from observed epidemiologic data collected in San Francisco and published data. Significant assumptions included 80% on ART would achieve viral load suppression, 100% reduction in HIV transmission with viral load suppression, 80% reduced infectivity in partially treated individuals, and 38% of MSM in the high behavioral risk group.   

Results:  All 3 treatment strategies resulted in reduction of new HIV infections compared to current practice. The percent reduction in incident infections seen with the 3 strategies was 49%, 71%, and 91% over the first 10 years for strategies 1, 2, and 3, respectively. The number of new infections averted by the 3 strategies is estimated at 3559, 5153, and 6560 at 10 years and 6146, 10512, and 13729 at 20 years. HIV prevalence is predicted to decline for all three strategies as seen in the figure below:

Conclusion:  In a city with a mature HIV epidemic where the majority of individuals have known HIV status, the successful application of more widespread ART would result in large numbers of HIV incident cases averted and decline in prevalence when combined with annual HIV testing to detect new cases.