Background:  The likelihood that persons living with HIV/AIDS (PLWHA) will transmit the virus varies based on their stage, awareness of infection, and whether they are on antiretroviral therapy (ART). At a population level, the relative contributions of groups defined by these factors to overall HIV transmission will vary based on the population’s testing frequency and ART use. 

Methods:  We developed a simple, cross-sectional mathematical model of HIV transmission in MSM that includes ART use, mean time from infection to HIV diagnosis, relative measures of behavioral risk, and transmissibility. Base case epidemiologic parameters were derived from surveillance data from King County, WA. Biological parameters were taken from the scientific literature. In our base case scenario, we assumed that PLWHA decrease their behavioral risk 70% following diagnosis, that primary HIV is 8 times as transmissible as chronic infection, and that ART decreases transmissibility by 90%.

Results:  Based on King County data, we assumed that the mean time from HIV infection to diagnosis is 15 months; HIV incidence and prevalence are 1% and 15%, respectively; and 70% of diagnosed persons are on ART; 7% of HIV infections would be undiagnosed. In addition, 31% of infections would be transmitted by persons with primary HIV; 26% by persons with chronic, undiagnosed HIV; 35% by diagnosed persons off-ART, and 8% by diagnosed persons on ART. The proportion of infections attributable to chronic, undiagnosed infection increases with longer periods from infection to diagnosis; increasing this parameter from 15 to 24 months increases the proportion of infection transmitted from persons with undiagnosed chronic infection from 26% to 37%, and decreases the proportion transmitted from diagnosed persons off ART from 35% to 29%. Decreasing the proportion of diagnosed persons on ART from 70% to 50%, increases the proportion of all infections transmitted by diagnosed persons off ART from 35% to 48%.

Conclusions:  A majority of new HIV infections among MSM are probably transmitted by persons who are unaware of their HIV status, but approximately half of these cases are likely attributable to primary HIV. Estimates are sensitive to parameters that are not known with precision and probably vary by population. Under plausible scenarios, a sizable proportion of infections are transmitted from diagnosed persons off ART, infections that could be reduced through more widespread treatment.