The level of HIV-1 in the blood of an HIV-infected partner is the
single most important factor influencing risk of sexual transmission to
an uninfected partner, according to a multinational study of
heterosexual couples in sub-Saharan Africa. The study, published in the Journal of Infectious Diseases,
calculated the risk of HIV-1 transmission per act of sexual intercourse
and found the average rate of infection to be about 1 per 900 coital
acts. The findings also confirmed that condoms are highly protective and
reduce HIV infectivity by 78 percent.
James P. Hughes, PhD, and colleagues at the University of Washington
and the Fred Hutchinson Cancer Research Center, in Seattle; the
University of Witwatersrand in South Africa; the University of Nairobi
and Kenyatta National Hospital, in Kenya; and the Rwanda-Zambia HIV
Research Group conducted a study that included 3,297 HIV-discordant
couples (where one person is HIV-infected, and the other is not) in
eastern and southern Africa who were enrolled in a randomized trial of
acyclovir suppressive therapy. The couples had frequent follow-up to
measure plasma HIV-1 RNA in the infected partner and genetic testing to
link the transmitted virus to the index HIV-infected partner, to prevent
inclusion of infections acquired from other possible partners. HIV
acquisition was not affected by the acyclovir therapy.
The study confirmed that condoms are highly protective, reducing the
risk of HIV transmission by 78 percent when subjects reported using a
condom. Most important, the authors noted, was the level of HIV-1 RNA in
the blood of the infected partner. The higher the viral load in the
index infected partner, the higher the risk of transmission, emphasizing
the importance of lowering viral load to help prevent the spread of
HIV-1 through sex. Older age was associated with reduced transmission
per sex act, and male circumcision reduced female-to-male transmission
by approximately 47 percent. Genital herpes infections and the presence
of genital ulcers were associated with increased rates of transmission.
"Our results underscore the importance of antiretroviral therapy,
and, possibly, treatment of co-infections, to reduce plasma HIV-1 viral
load in HIV-1 infected partners, and condom promotion, male
circumcision, and treatment of symptomatic sexually-transmitted
infections for HIV-1 uninfected partners as potential interventions to
reduce HIV-1 transmission," the authors wrote.
The findings also showed that the risk of an HIV-infected man
transmitting an infection to a woman not infected with HIV was about
twice the risk of an HIV-infected woman transmitting to an
HIV-uninfected man. However, this difference can be attributed to the
difference in viral loads between men and women, the authors noted. On
average, HIV-infected men have higher HIV-1 loads. Difference in age and
having genital herpes in the HIV-uninfected partners also help account
for the disparity—the HIV-uninfected female partners were, on average,
younger and had higher rates of genital herpes than their male
counterparts.
Previous studies examining HIV-1 per-act infectivity have been
significantly smaller and not as comprehensive in terms of measuring
plasma HIV-1 RNA and the use of genetic linkage of transmissions. In an
editorial commentary, Ronald H. Gray, MD, and Maria J. Wawer, MD, both
of Johns Hopkins University in Baltimore, noted that Dr. Hughes and his
colleagues have possibly recorded the most precise estimates of HIV-1
transmission per sexual act during latent HIV disease, providing a
valuable addition to knowledge in this area, where much remains to be
learned.
Additional research using the genetic data collected from this
study, in addition to new data from another recently completed clinical
trial, is planned to help explain the variation in transmission risk
among couples, the authors noted.
Fast Facts:
1. In HIV-discordant couples—where one partner is infected
with HIV, and the other is not—viral load of the infected partner was a
major factor affecting the HIV transmission rate.
2. Condom use among HIV-discordant couples was 78 percent effective in preventing transmission to the uninfected partner.
3. Factors such as age, male circumcision status, and sexually transmitted infections also affected transmission probability.
