What Is the Risk of HIV From Vaginal Sex?

Multiple vulnerabilities increase risk in men and women

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Giovanni Dall'Orto

Vaginal sex is one the primary ways a person can become infected with HIV. It accounts for many of the nearly 7,500 new infections among women each year in the U.S. and nearly 1,000 new infections among heterosexual men.

Globally, the figures are even more dismaying. While the sexual transmission of HIV in the U.S. is highest among gay and bisexual men (representing 63 percent of all new infections), heterosexuals are by far the group most affected worldwide.

This is especially true in Africa where as many as every four or five new infections are among heterosexuals. In these populations, vaginal sex is the predominant route of infection.

HIV Risk by Sexual Activity

When discussing HIV risk, people often try to ascertain which "type" of sex is riskier; vaginal, anal, or oral. From a purely statistical standpoint, anal sex is considered the highest risk activity with an 18-fold greater risk of infection compared to vaginal sex.

But this assessment is somewhat misleading, at least from an individual perspective. While vaginal may pose a “lower” risk comparatively, the figures neither take into account the way in which the disease is distributed between men and women nor the vulnerabilities which place some individuals at extremely high risk of infection.

Consider, for example, that women are anywhere from three to four times more likely to get HIV from men than the other way around.

Or that young women are far more likely to get HIV from her first sexual encounter than a male partner.

Conversely, there are some men who are far more likely to get HIV than others. Studies have shown, for example, that uncircumcised men are more than twice as likely to get HIV after vaginal sex than circumcised men.

Vulnerabilities vary by individual, so assessing what the real risk of vaginal sex requires a better understanding of the factors that place some women and men at greater risk than others.

Vulnerabilities That Can Increase HIV Risk in Women

The risk of HIV from unprotected vaginal sex is higher among women for a number of reasons. From a physiological standpoint, the tissues of the vagina (epithelium) are far more susceptible to HIV than those of the penis.

HIV is able to pass through these tissues when the immune system recognizes the invading virus and send defensive cells (called macrophages and dendritic cells) to "grab and drag" them through the lining to be destroyed. Instead, HIV turns the table and attacks the very cells (called CD4 T-cells) meant to help neutralize them. By doing so, the body helps facilitate its own infection.

And, because the surface area of the vaginal epithelium is far greater than that of the male urethra, the opportunity for infection is increased, often exponentially.

Other physiological vulnerabilities include:

  • Cells beneath the surface of the cervix are especially vulnerable to HIV, particularly during adolescence, a woman's first pregnancy, or in the presence of a sexually transmitted infection (STI) like chlamydia or human papillomavirus (HPV).
  • Women with a genital tract infection, whether bacterial, viral, or fungal, are at increased risk. Some studies have suggested that bacterial vaginosis is associated with an eight-fold increase in risk. This translates into a one in 100 chance of getting HIV during vaginal intercourse.
  • The duration of exposure and volume of infected fluid are also key factors in determining whether a person gets infected or not. As such, unprotected sex can increase HIV risk in a woman if the man ejaculates into her vagina.
  • Open sores or ulcers from STIs like syphilis can increase risk in both men in women. In women, however, the sores are often internalized and unnoticed.
  • While the daily use of an HIV drug called pre-exposure prophylaxis (PrEP) can dramatically decrease the risk of HIV in an uninfected partner, there is evidence that works less well in women. Research suggests the level of the active drug molecule in vaginal tissue isn't near as high as in rectal tissue.
  • Douching practices may also alter the vagina's "good" bacterial flora, although this is still under debate.

None of this, of course, takes into account any of the social vulnerabilities that can place women at increased risk. These include sexual violence in relationships which not only steals a woman's chance for self-protection but can result in damage to delicate vaginal tissue.

Poverty, social norms, and gender imbalances further ensure that any dominance a man might have outside the bedroom will extend into the bedroom, as well. All of these contribute to higher rates of HIV in women.

Vulnerabilities That Increase HIV Risk in Men

The fact that men are less susceptible to HIV than women shouldn't underplay the fact that they also have vulnerabilities that can increase their personal risk of infection.

We know, for example, that an uncircumcised penis can facilitate infection due to the bacteria-rich environment beneath the foreskin. In response, the body will produce a type of dendritic cell (called Langerhans cells) to help control the bacteria.

When a man has unprotected sex with an HIV-positive woman, those same cells can "grab and drag" the virus them through the tissue barrier and present them to CD4 T-cells to infect. STIs and genital tract infections further increase HIV risk.

From a cultural standpoint, society’s definition of masculinity can often normalize sexual adventure in men and even encourage it. It creates a double standard that can place a man at greater risk of HIV by associating virility with multiple partners or other high-risk behaviors.

Vulnerabilities in Both Men and Women

There are vulnerabilities that increase the likelihood of infection in both men and women. Among them:

  • Any increase in the infected partner’s viral load (the volume of virus in blood) increases the risk to the uninfected partner. Studies have shown that every one-log increase in viral load — from, say, 100 to 1,000 — can double and even triple the risk of infection.
  • Additionally, a high viral load during acute infection (the stage immediately following exposure) is associated with a 28-fold increase in risk. This translates to a one in 50 chance of getting HIV if you’re a woman and a one if 500 chance if you’re a man.
  • Drinking alcohol or taking drugs can lower inhibitions and affects a person's ability to make safe choices, such as using condoms or remaining adherent to HIV drug therapy.

Per-Exposure Risk by Vaginal Sex

From the perspective of "per-exposure" risk (the chance of getting HIV from a single sexual act), risk can vary based on gender, the viral load of the HIV-positive partner, and even the part of the world you live in.

These figures do not take into account any other factors that can increase risk, including the presence of an STI, injecting drug use, or a co-existing infection like hepatitis C.

Exposure

Exposure Type

Per-Exposure Risk

Vaginal

Vaginal sex, female-to-male (high-income country)

0.04% (one in 2500)

 

Vaginal sex, male-to-female (high-income country)

0.08% (one in 1250)

 

Vaginal sex, female-to-male (low-income country)

0.38% (one in 263)

 

Vaginal sex, male-to-female (low-income country)

0.3% (one in 333)

 

Vaginal sex, asymptomatic HIV

0.07% (one in 1428)

 

Vaginal sex, late-stage symptomatic HIV

0.55% (one in 180)

Accidental Exposure and Vaginal Sex

If you believe you may have been exposed to HIV, either through a condom burst of condomless anal sex, there are medications that can greatly reduce the risk of infection called post-exposure prophylaxis (PEP).

PEP consists of a 28-day course of antiretroviral drugs, which must be taken completely and without interruption. In order to minimize the risk of infection, PEP must be started as soon as possible​ — ideally within one to 36 hours of exposure.

A Word From Verywell

Assessing your personal risk for HIV should never be a numbers game. Whether the odds are one in 10 or one in 100,000, it’s important to remember that you can get HIV after just one exposure.

Explore all of the available tools of HIV prevention. In addition to PrEP, a strategy which can reduce HIV risk by 76 percent, an undetectable viral load in the HIV-positive partner can further lower risk by 96 percent or more. And don't forget the tried-and-true condom, the consistent use of which is associated with a 20-fold decrease in risk.

By formulating a holistic approach to prevention, you can continue to enjoy a healthy sex life while protecting yourself or a loved one from the risk of HIV.

Sources:

Boily, M.; Baggaley, R.; Wang, L.; et al. "Heterosexual risk of HIV-1 infection per sexual act: Systematic review and meta-analysis of observational studies." Lancet Infectious Diseases. February 2009; 9(2):118-129.

Centers for Disease Control and Prevention (CDC). "Today’s HIV Epidemic." Atlanta, Georgia; August 2016.

Cohen, C.; Lingappa, J.; Baeten, J.; et al. "Bacterial vaginosis associated with the increased risk of female-to-male HIV-transmission: a prospective cohort analysis among African couples." PLoS Medicine. June 2012; 9(6):e1001251.

Hollingworth, T.; Anderson, R.; and Fraser, C. "HIV-1 transmission, by stage of infection." Journal of Infectious Diseases. September 1, 2008; 198(5):687-693.

United Nations Programme on HIV/AIDS (UNAIDS). "Global AIDS Update 2016." Geneva, Switzerland; 2016:9.

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