Transmission

In 2007, the most recent year for which figures are available in the US, although women of colour accounted for roughly 25% of the female population, 83% of all women diagnosed with AIDS were Black or Hispanic. This disproportionate effect suggests that effective intervention strategies to lessen HIV transmission will need to address contributing co-factors in specific communities.

Pooling data from studies in high-income countries, researchers calculated the risk of transmission from an HIV-positive man to a female partner was 0.08% per sexual act, that is, one infection per 1250 sexual acts. When the female partner was HIV-positive, the male partner’s risk of acquiring HIV was 0.04% per sexual act, or 1/2500.¹ 

This analysis, as well as an earlier meta-analysis, concluded that the multiplicity of co-factors affecting transmission make it extremely difficult to generalise about HIV infectivity risk. Focusing on a single figure as the per-act risk of transmission is misleading and can lead to underestimation of transmission risk. Even though co-factors confound attempts to estimate risk, they are valuable in building HIV epidemic models and in identifying optimal intervention strategies. The basic, but excellent, point is made that every HIV infection results from one transmission event.²

Setting aside economic, social, and cultural underpinnings for the increasing incidence of HIV infection in women, HIV transmission is through well-established routes that include unprotected vaginal or anal sex, use of contaminated injecting equipment, being the recipient of contaminated blood or blood products in medical settings, or through transmission from an HIV-positive mother.

A woman's risk of acquiring HIV is affected by the use (and efficacy) of barrier contraception, an existing sexually transmitted infection, inflammation or disruption of genital or rectal mucosa, and immunosuppression for any reason. A number of studies have concluded that male circumcision does not have a direct effect on reduced HIV transmission or transmission of other sexually transmitted infections (STIs) to women.³

A shocking finding presented in 2009 was that the diagnosis of a sexually transmitted disease carries a threefold higher risk for a subsequent HIV diagnosis within 10 years in young women. More than 48,000 HIV-negative women diagnosed with an STI between the ages of 13 and 24 were followed in this study. There was also a significant trend for women first diagnosed with gonorrhoea or syphilis to be later diagnosed with HIV infection than women whose first STI was chlamydia.⁴

The interplay between the female endocrine and immune system, hormonal contraception, and time of life (i.e. prepubescent, menopausal) is being examined to see if there is a dynamic effect on transmission efficiency and/or disease progression. As estradiol and progesterone levels affect ectocervical and vaginal secretions, it has been suggested that at some point in the menstrual cycle, natural immunity against infection is suppressed.^{5 6}