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In addition, UCSF researchers found that male-to-female transmission rates continue to remain extremely low, about 9 out of every 10,000 unprotected sexual encounters, and even lower for female-to-male infections. Females are about eight times more likely than males to become infected by their HIV-positive partners, said study principal investigator Nancy Padian, Ph.D., UCSF assistant professor of obstetrics, gynecology and reproductive sciences.
"We now know many of the risk factors that affect the likelihood of transmission between infected individuals and their heterosexual partners," she said. "Elimination or modification of these factors would result in reduced transmission of HIV."
Final results of the long-term UCSF study of Northern California heterosexual couples with one HIV-infected partner are published in the current (August) issue of the American Journal of Epidemiology.
From 1985 to 1995, Padian and her research colleagues enrolled 82 HIV-infected women and their male partners and 360 HIV-infected men and their female partners into a study to examine rates of and risk factors for heterosexual transmission of HIV. Study participants were interviewed and given physicals at the start of the study and every six months. The couples were also counseled during each visit about safe sexual practices, and study staff were available at any time by phone.
While no new HIV transmissions occurred during the course of the study, 68 women and two men were infected by their HIV-positive partners prior to the start of the study.
The UCSF study showed that a history of STDs and injection drug use greatly increased HIV transmission risk from the HIV-positive male to his female partner. Of the 77 HIV-positive male injection drug users, 22 percent transmitted the virus to their partners. Of the 163 women who had an STD, 25 percent became infected.
Only two men had become infected by their HIV-positive female partners. In the first infection, both partners reported several instances of post-coital bleeding from the genital area. In the second case, the woman appears to have infected her partner with both HIV and chlamydia within a short period.
"Because there were only two instances of female-to-male transmission, we could not examine risk factors for these events statistically," Padian said. "However, the fact that chlamydia was transmitted simultaneously with HIV is striking."
Researchers observed dramatic behavior changes during the course of the study. At the start of the study, no couple abstained from sex, only 32.2 percent ever used a condom, and 37.9 percent had anal sex. At the final follow-up visit 10 years later, 14.5 percent abstained from sex, 74 percent used condoms, and only 8.1 percent had anal sex.
The fact that no transmission occurred among the 25 percent of couples who did not consistently use condoms shows that there is very low infectivity among heterosexual couples with one HIV-positive partner, according to the UCSF study. Infectivity increases greatly with injection drug use and the presence of an STD, as well as other factors such as risky sexual practices and lack of condom use.
Other similarly conducted studies have reported higher rates of heterosexual transmission, particularly from females to males, Padian said, adding that geographic differences in sexual practices and co-factors for infection (such as intrauterine contraceptive devices) and misclassification of transmission due to other sources (such as needle sharing) may explain the UCSF study's lower transmission rates.
Co-authors of the UCSF study are Stephen C. Shiboski, Ph.D., UCSF assistant professor of epidemiology and obstetrics; Sarah O. Glass, UCSF research specialist; and Eric Vittinghoff, Ph.D., San Francisco Department of Public Health.
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