Vaginal intercourse

Unprotected vaginal intercourse is a well-established high-risk route of sexual HIV transmission. Worldwide, the majority of new HIV infections occur as a result of unprotected penile-vaginal intercourse.

Unprotected vaginal intercourse is a highly efficient route of HIV transmission because high concentrations of HIV can occur in semen and vaginal fluids , and because the genital tissues are very susceptible to infection. HIV is able to pass through the cells of the vaginal lining (epithelium) by migrating through specific cells and/or by passing through non-intact tissue. This allows the virus to reach the inner vaginal lining, which is rich in immune cells through which it can establish systemic infection.

Cells located beneath the surface of the cervix are particularly vulnerable to HIV infection,¹ especially during adolescence and during a woman's first pregnancy, or due to infection with human papilloma virus and chlamydia.

A meta-analysis of studies of heterosexual HIV transmission found that, in high-income countries prior to the introduction of combination therapy, the risk per sexual act was 0.04% if the female partner was HIV-positive, and 0.08% when the male partner was HIV-positive. However, these rates were considerably higher in lower-income countries, if the source partner was in either the very early or the late stage of HIV infection, or if one partner had genital ulcer disease.²

While women are at greater risk of infection from an HIV-positive male partner, unprotected vaginal intercourse is also high risk for men, because damaged penile tissue and the mucous membranes in the urethra and on the head of the penis – particularly underneath the foreskin – form a point of infection.

While research into vaginal microbicides and other protective strategies are ongoing, condoms are still the only proven means of reducing the risk of HIV transmission during vaginal intercourse. Many other factors affect the level of risk associated with vaginal intercourse, including the viral load of the infected partner, sexually transmitted infections in either partner, vaginal health, douching, circumcision and female genital mutilation. These factors are discussed at length in the chapter Co-factors that affect the risk of transmission.