By Jared Stern and Paula Cevaal
Globally, new HIV infections have been decreasing since the late 1990s and rates are currently sitting at 1.8 million infections each year – equating to 5000 new infections each day on top of the existing 37 million people living with HIV1. Although reductions in new HIV infections are certainly a commendable achievement, the current annual rate is far higher than the UNAIDS Fast-Track Cities target of 500,000 new infections per year by 20202. Additionally, the change in rates of new infections are different between different regions of the world, with greatest decreases since 2010 being seen in eastern and southern Africa3. Rather startling is the 60% increase in the number of new infections each year in Eastern Europe and Central Asia.
Just as there are differences in rates of HIV acquisition across the world, there are also differences in how most people acquire HIV 4,5. For instance, globally heterosexual individuals account for 56% of new HIV infections, however in sub-Saharan Africa this population accounts for 75% of new infections. Conversely, in low-prevalence regions like North America, Europe, and Australia, most new HIV infections occur among men who have sex with men, people who inject drugs, and sex workers and their clients. From these data, sexual intercourse clearly plays an important role in the HIV epidemic.
Whilst HIV transmission is largely attributable to sexual acts, when the risk of behaviours is estimated per exposure, the individual risk is quite low (Figure 1)6. Condom less anal intercourse has the highest risk of HIV transmission with 138 out of 10,000 sex acts resulting in HIV transmission to the receptive partner (often termed “bottom”) and 11 out of 10,000 sex acts resulting in HIV transmission to the insertive partner (“top”).
| Exposure route |
Risk per 10,000 exposures to an infected source |
Sexual exposurea
Receptive anal intercourse
Insertive anal intercourse
Receptive penile-vaginal intercourse
Insertive penile-vaginal intercourse
Receptive oral sex
Insertive oral sex |
138c
11d
8e
4e
Lowf
Lowf |
Table 1 – Estimated per-act probability of acquiring HIV from an infected source, by sexual exposure route. Adapted from Patel et al., Estimating per-act HIV transmission risk: a systematic review. AIDS. 2014<
Mucous surfaces or membranes line part or all of the vagina, rectum, and penis and are made up of one or more layers of densely packed cells, called epithelia, covered with fluid (mucous). The vagina is protected by multiple layers of cells whilst the rectal mucosal membrane is made up of just a single layer of cells. These mucous membranes act as barriers to protect against infections. For HIV to be transmitted from one person to another, the virus needs to encounter its target cells, predominantly CD4+ T-cells7,8. This can occur during condomless sex because there is an exchange of bodily fluids which may have HIV in them such as vaginal and rectal secretions, and semen (pre-cum and cum)9,10. Though the physical barriers make it difficult, HIV can pass through the mucous membranes of the vagina, rectum and penis to reach its target cells that it can infect. During sex, micro-tears in the vagina and rectum can occur, making it easier for HIV to pass through these barriers − especially in the rectum because of how thin the epithelium is, which explains why penile-anal intercourse has a higher risk of transmission than penile-vaginal intercourse.
Also contributing to increased chance of transmission via the rectum is the fact that it is lined with immune cells that can either be infected themselves or present the virus to other immune cells that can be infected11. Similarly, the foreskin has many immune cells such as Langerhans cells (a cell specialised in taking up foreign particles like viruses) that can be infected by HIV which may explain why penile circumcision has been shown as an effective method to lower the chances of transmission12.
Like the micro-tears during sex, open wounds at mucosal sites provide a way for HIV to bypass the barriers – a factor that could increase the otherwise negligible chance of HIV transmission via oral sex. This is also partly why having other sexually transmitted infections (STIs) can increase the risk of HIV transmission; STIs can result in open wounds (e.g. herpes or syphilis) or inflammation of mucosal sites13,14. Another factor affecting the likelihood of transmission is an HIV-positive person’s viral load: if there is more virus present in their body and bodily secretions, there is more virus their sexual partner will be exposed to15,16. This is also linked with disease progression, as individuals experiencing acute (recent) infection or late-stage disease have higher viral loads17. Reducing one’s viral load to undetectable levels by taking antiretroviral therapy (ART) has been shown to eliminate any sexual transmission of HIV to another person18, 19,20.
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