Vaccine and Infectious Disease Division

Calculating HIV’s infectivity

Understanding the likelihood of HIV transmission per sexual act is important in designing effective prevention methods and in understanding how HIV spreads through a population. Past studies to calculate this transmission risk have come up with very different numbers, perhaps in part because the studies were small and volunteers were not always able to accurately recount sexual events over a long period of time. The University of Washington’s Partners in Prevention study, a study of more than 3,400 HIV serodiscordant heterosexual couples in sub-Saharan Africa, offered a unique opportunity to better calculate HIV’s transmission rate using a very large sample and adjusting for confounding factors such as HIV viral load, co-infection with herpes simplex virus-2 (HSV-2) and other viruses, and condom usage.

VIDD member Dr. James Hughes led the analysis of the study’s data. Like past studies, their initial results found that women were more likely to be infected by men than the reverse, but once the researchers factored in HIV viral loads, HSV-2 infection and age this disparity disappeared. That is, the difference in transmission between men and women may be largely due to the fact that, on average, HIV positive men have higher viral loads. They also found that reported condom usage decreased the chance of transmission by 78 percent. The group’s results point to the importance of reducing HIV viral load and education on means to prevent transmission, such as condom usage and male circumcision. –RT

Hughes JP, Baeten JM, Lingappa JR, Magaret AS, Wald A, de Bruyn G, Kiarie J, Inambao M, Kilembe W, Farquhar C, Celum C; Partners in Prevention HSV/HIV Transmission Study Team. Determinants of per-coital-act HIV-1 infectivity among African HIV-1-serodiscordant couples. J Infect Dis. 2012 Feb;205(3):358-65.