Background:  We hypothesized that the decrease in community viral load (CVL) in San Francisco from 2002 to 2008 would be associated with a reduction in new HIV infections.

Methods:  We used San Francisco’s comprehensive HIV/AIDS surveillance system, which includes mandatory laboratory reporting of viral loads, to calculate 2 distinct measures of CVL (mean and total), and to examine trends in CVL overall and within different populations. We defined mean CVL as the mean of the most recent viral load of all reported HIV⁺ individuals in a particular population, divided by the number of reported HIV⁺ individuals in the population. Total CVL is the sum of the most recent viral loads of all HIV⁺ individuals in a particular population. Outcomes included annual numbers of new HIV diagnoses (from 2004 to 2008), changes in estimated HIV incidence from 2006 to 2007. To assess the relationships of CVL with these outcomes, we first plotted them against CVL, and then used Poisson models with robust standard errors for inference.

Results:  Both measures of CVL (mean and total) decreased from 2002 to 2008 and were accompanied by decreases in new HIV infections, as measured by both new diagnoses of HIV cases and HIV incidence. The trend in mean CVL (Figure 1) was significantly associated with newly diagnosed HIV cases from 2004 to 2008 (P =0.003) and HIV incidence from 2006 to 2007 (P <0.0005). Total CVL was also associated with newly diagnosed HIV cases (P =0.002) and HIV incidence (P <0.0005).

Conclusions:  Increased antiretroviral treatment options and coverage, as well as increasing HIV status awareness, may have led to decreases in CVL in San Francisco from 2002 to 2008 with subsequent decreases in HIV diagnoses and, though limited by only 2 years of data, with fewer new HIV infections. Findings support the hypothesis that wide-scale early ART can have a preventive effect at a population-level. Because CVL is temporally upstream of new HIV infections, public health departments should consider adding CVL to routine HIV surveillance to track the epidemic and evaluate the effectiveness of HIV prevention and treatment interventions.