The uninfected partners were tested periodically to see whether they had acquired HIV. The researchers used genetic testing of the virus to confirm that any new HIV infections had been acquired from the study partner designated at the study’s start.
Eighty-sixHIV transmissions occurred during the study period.
Men were about twice as likely to transmit HIV to women as women were to men. This increased risk of transmission could be attributed to higher virus concentrations in the blood of men compared with women, according to the study. In addition, women were more likely to have genital herpes, which increases susceptibility to HIV.
Why are women more easily infected by HIV than men?
Women are two to four times more likely to get infected with HIV through unprotected vaginal sex than men, due to the following reasons:
- Women, as the recipients of semen, are exposed to semen for a longer time (while semen remains in the body of a woman for a few hours, a man is exposed to the body fluids of a woman for only a short time);
- The concentration of HIV in semen is much higher than the concentration of HIV in vaginal fluids;
- Women possess a larger surface area of mucosa (the thin lining of the vagina and cervix) which is exposed to their partner’s secretions during sexual intercourse;
- Many women have cervical or vaginal conditions, such as STIs, erosions, open sores and infections, that facilitate the transmission of HIV;
- Many women practise “dry” sex and they cause damage or infection to the vaginal walls (“dry” sex is a cultural practice where women use herbs or other substances - such as Jik or washing powder - to dry out their vaginas for the benefit of some men who believe that a dry vagina is a sign of faithfulness, or to heighten their sexual pleasure.) This practice is painful and extremely dangerous because it increases the risk of HIV infection;
- Transmission of HIV is more likely to occur just before, during or immediately after menstruation because of the large, raw area of the inner uterine lining that is exposed;
- Younger women are especially vulnerable to HIV infection because their genital tracts are not yet fully mature, their vaginal secretions are not so copious, and because they are more prone to lacerations or tears of the vaginal lining. (There is also evidence to suggest that women once again become more vulnerable to HIV infection after menopause).
- Women often practice anal sex to avoid pregnancy, to maintain “virginity”, or because their male partners prefer anal sex for reasons similar to the reasons why they prefer “dry sex”. Research has shown that the chances to become infected with HIV after one act of unprotected receptive anal sex is approximately 20 times greater than after one act of unprotected vaginal sex.
- Social inequalities often make women more vulnerable to HIV infection especially in societies which accord women a lower status than men. Women in such situations have little or no control over their sex lives, and they are not in a position to negotiate safer sex practices because they fear violence and abandonment should they try to do so. Women from low socio-economic environments are often driven to prostitution, and they are particularly vulnerable to rape.
Condoms were reported to be used in 93 percent of sexual acts, but the researchers suspect their use was overreported. Therefore, condoms actually may be even more effective at preventing HIV transmission than the 78 percent reduction that the researchers estimated, Hughes said.
The AIDS epidemic
The study relied on self-reports, which might be wrong. However, errors in reports of the number of sexual acts would be unlikely to affect most of the study results, Hughes said.
HIV infection is best characterized as a sexually transmitted disease (STD). In the United States, ∼75% of HIV-infected individuals acquired the virus through sexual activity. It is estimated that, in other areas of the world, >90% of new infections among adults are acquired via sexual activity. Effective methods to prevent sexual transmission of HIV are critical to the effort to defeat the worldwide epidemic. Although injection drug use continues to be an important source of transmission in the United States (it ranks second, after sexual activity), other avenues of acquiring infection, including vertical transmission from mother to child and use of HIV-contaminated blood products, are of diminishing importance.
In the United States, male-male sex is the avenue of HIV transmission in ∼60% of new cases. Heterosexual contact, although accounting for only ∼17.5% of new infections, is the mode of HIV transmission in 80% of new cases in women. From 2000 through 2003, the estimated number of HIV/AIDS cases increased by 5% among males and decreased by 2% among females. In 2003, a total of 72% of all HIV/AIDS cases were diagnosed in men or adolescent boys . The difference in the HIV infection rate between men and women in the United States, which has persisted since the start of the epidemic, may be best understood in the context of the efficiency of different routes of HIV transmission, particularly the relative rates of transmission via vaginal and anal intercourse.
Revision date: June 18, 2011
Last revised: by Sebastian Scheller, MD, ScD