Decreases in community viral load are accompanied by reductions in new HIV infections in San Francisco

Abstract

Background: At the individual level, higher HIV viral load predicts sexual transmission risk. We evaluated San Francisco's community viral load (CVL) as a population level marker of HIV transmission risk. We hypothesized that the decrease in CVL in San Francisco from 2004-2008, corresponding with increased rates of HIV testing, antiretroviral therapy (ART) coverage and effectiveness, and population-level virologic suppression, would be associated with a reduction in new HIV infections.

Methodology/principal findings: We used San Francisco's HIV/AIDS surveillance system to examine the trends in CVL. Mean CVL was calculated as the mean of the most recent viral load of all reported HIV-positive individuals in a particular community. Total CVL was defined as the sum of the most recent viral loads of all HIV-positive individuals in a particular community. We used Poisson models with robust standard errors to assess the relationships between the mean and total CVL and the primary outcome: annual numbers of newly diagnosed HIV cases. Both mean and total CVL decreased from 2004-2008 and were accompanied by decreases in new HIV diagnoses from 798 (2004) to 434 (2008). The mean (p = 0.003) and total CVL (p = 0.002) were significantly associated with new HIV cases from 2004-2008.

Conclusions/significance: Reductions in CVL are associated with decreased HIV infections. Results suggest that wide-scale ART could reduce HIV transmission at the population level. Because CVL is temporally upstream of new HIV infections, jurisdictions should consider adding CVL to routine HIV surveillance to track the epidemic, allocate resources, and to evaluate the effectiveness of HIV prevention and treatment efforts.

Figure 1. Spatial Distribution of CVL by Neighborhood, 2005–2008.

Neighborhood mean (1a) and total CVL (1b) are shown. Mean CVL was highest among homeless individuals (38,974copies/mL; N = 775; 6%). The highest mean CVL (38,428 copies/mL; N = 278; 2%) was in the southeast neighborhood of Bayview, which is characterized by lower income and a predominantly African-American population. The northeast and inner city areas of the Tenderloin and South of Market (characterized by low income and large numbers of IDU, commercial sex workers, and transgendered persons) also had a mean CVL above the municipal average (28,093 copies/mL; N = 1,486; 12%). The Castro neighborhood (an historic gay and relatively upper-income neighborhood with very high HIV/AIDS case density) had a mean CVL of 21,352 copies/mL (N = 2,106; 17%), below the city as a whole. The distinction between mean CVL and total CVL is seen in 1b. The highest total CVLs are evident in the Tenderloin, South of Market, Mission, and the Castro, where there are either large numbers of persons living with HIV, many persons with high VLs, or a combination thereof.

Figure 2. Total CVL and Virologic Suppression, 2004–2008.

There was a statistically significant decline in annual measures of total CVL from 2004–2008 (p = 0.021). As expected, there was an inverse correlation between the increase in virologic suppression (red line) from 45% in 2004 to 78% in 2008 (p = 0.006) and total CVL (p = 0.011).

Figure 3. Mean CVL and New HIV Infections, 2004–2008.

There was a statistically significant decline in annual measures of mean CVL from 2004–2008 (p = 0.037). Newly diagnosed cases of HIV (shown in red with ) decreased in San Francisco from 798 (2004) to 434 (2008) (p<0.005). The point estimates of HIV incidence (shown in dark red with ▵) using the CDC methods also declined from 935 [95% CI 658–1212] in 2006, to 792 [552–1033] in 2007 and 621 [462–781] in 2008, although the change was not statistically significant (trend p = 0.29). The reductions in annual measures of mean CVL were significantly associated with decreases in newly diagnosed and reported HIV cases from 2004–2008 (p = 0.003). Longitudinal reductions in estimated HIV incidence were consistent with the trends in mean and total CVL, but the association in the meta-regression was not statistically significant (p>0.3).