- Exploring HIV Transmission Rates
- HIV Transmission
- HIV Must Be Present
- There Needs to Be Enough Virus
- HIV Must Get Into the Bloodstream
- HIV Transmission Routes
- Sexual Transmission
- Non–sexual Transmission
- Partners Living with HIV
- When One Partner Is HIV Positive
- Loving Someone Who Has HIV
- Protecting Yourself and Your Partner
- Getting Comfortable Disclosing Your Status
- Protecting Others
- My viral load is not undetectable. How can I prevent HIV transmission?
Vaginal sex and the risk of HIV transmission
- Key points
- Recent infection
- Sexually transmitted infections
- Male circumcision
- Bacterial vaginosis
- Withdrawal before ejaculation
- Vaginal dryness
- How you can reduce the risk
- Challenges in calculating a number
- All exposures are not equal
- Interpreting the numbers—what additional information needs to be provided?
- Differences in risk
- How Can You Prevent Getting HIV from Anal or Vaginal Sex?
- Is Abstinence an Effective Way to Prevent HIV?
- If You Are Living with HIV, How Can You Prevent Passing It to Others?
- Learn More
Exploring HIV Transmission Rates
Having sex with a person living with HIV increases the risk of contracting the virus. HIV may be transmitted both anally and vaginally during sexual intercourse. According to the CDC, the risk of transmission for receptive penile-vaginal sex is 8 per 10,000 exposures. For insertive penile-vaginal sex, the risk of transmission lowers to 4 out of 10,000 exposures.
Receptive anal intercourse with a partner who is HIV-positive is the sex act that’s most likely to transmit the virus. For every 10,000 instances of receptive anal intercourse with a partner who has HIV, the virus is likely to be transmitted 138 times.
Insertive anal intercourse poses a lower risk, with 11 transmissions per 10,000 exposures. All forms of oral sex are considered low risk. Biting, spitting, throwing bodily fluids, and sharing sex toys all have such a low risk of transmission that the CDC considers the risk “negligible.”
How to practice safe sex
Using condoms regularly and correctly is the best way to prevent the transmission of HIV and other sexually transmitted infections. Condoms act as barriers against semen and vaginal fluids. Always use latex condoms — never use lambskin or homemade condoms, which offer little to no protection.
Still, even sex with a condom is not 100 percent risk-free. Misuse and breakage can be problems. People who are sexually active should consider getting HIV tests along with other STI testing. This can help each person understand the risk of transmitting or contracting the virus.
If one person has HIV and the other doesn’t, the CDC reports that using condoms alone may lower the risk of contracting the virus by 80 percent.
For people who do not have HIV who have a sexual partner living with HIV, the use of pre-exposure prophylaxis (PrEP) can help reduce the risk of transmission through sex. When used daily along with other preventive measures, PrEP can reduce the risk of transmission by as much as 92 percent, according to the CDC.
Living with HIV and taking antiretroviral therapy can reduce the risk of transmission by up to 96 percent. Combining condoms with antiretroviral therapy can provide even more protection. Possible exposure may also be remedied with post-exposure prophylaxis (PEP) treatment.
According to the WHO, this approach includes a combination of:
- HIV testing
- 28-day course of antiretroviral therapy for HIV
- follow-up care
It’s important to note that antiretroviral therapy as a part of PEP treatment for HIV is most effective when started within 72 hours of exposure to HIV.
Understanding how HIV can and cannot be transmitted is vital to preventing new infections. HIV is a rapidly changing virus but, thankfully, it is also entirely preventable. Below are some key facts to help you learn more about how HIV is transmitted and how to reduce your, or others’, risk of being infected.
HIV Must Be Present
You can only become infected with HIV if someone involved in an exposure situation is already infected with HIV. Some people assume that certain behaviors or exposure situations can cause HIV disease, even if the virus is not present. This is not true.
There Needs to Be Enough Virus
The concentration of HIV determines whether infection will occur. In blood, for example, the virus is very concentrated. A small amount of blood is enough to infect someone. The concentration of virus in blood or other fluids can change, in the same person, over time. Persons who take HIV medications as prescribed can have very low quantities of HIV present in bodily fluids, greatly reducing the risk of transmitting HIV to their partners.
It is important to note that HIV is a very fragile virus which will die quickly when exposed to light and air. Exposure to small amounts of dried blood or other infectious fluids is not a realistic risk for HIV transmission.
HIV Must Get Into the Bloodstream
It is not enough to be in contact with an infected fluid for HIV to be transmitted. Healthy, intact skin does not allow HIV to get into the body.
HIV can enter through an open cut or sore, or through contact with mucous membranes. Transmission risk is very high when HIV comes in contact with the more porous mucous membranes in the genitals, the anus, and the rectum which are inefficient barriers to HIV. Although very rare, transmission is also possible through oral sex because body fluids can enter the bloodstream through cuts in the mouth.
HIV can be transmitted from an infected person to another through the following infectious fluids:
- Semen (including pre–seminal fluid)
- Vaginal secretions
- Rectal fluids
- HIV can also be transmitted through breast milk expressed through feeding, in limited circumstances, where there is exposure to large quantities.
HIV Transmission Routes
HIV can enter the body through open cuts or sores and by directly infecting cells in mucous membranes. HIV cannot cross healthy, unbroken skin. Unprotected sexual intercourse (oral, vaginal, and anal), sharing needles for injection drug use, and mother to child transmission (in utero, during delivery, and breastfeeding) are the main transmission routes for the HIV virus.
Sexual activity is the most common way HIV is transmitted. HIV can be transmitted through sexual intercourse, both vaginal and anal. HIV can easily pass through the mucus membranes in the genitals and the rectum, or may pass through cuts and sores.
Although very rare, HIV can also be transmitted through oral sex. Conditions such as bleeding gums and poor oral health increase the risk of transmission through oral sex.
Anal sex without a condom is the riskiest sexual activity for HIV transmission. The receptive partner is at the greatest risk because anal tissue is easily bruised or torn during sex which then provides easy access to the bloodstream for HIV carried in semen. The insertive partner is also at some risk because the membranes inside the urethra can provide entry for HIV into the bloodstream. The presence of other sexually transmitted infections can increase the risk of HIV transmission during anal sex.
Unprotected vaginal sex is also considered risky for HIV transmission. The receptive partner is at the greatest risk because the lining of the vagina is a mucous membrane which can provide easy access to the bloodstream for HIV carried in semen. The insertive is also at some risk because the membranes inside the urethra can provide an entry for HIV into the bloodstream. The presence of other sexually transmitted infections can increase the risk of HIV transmission during vaginal sex.
Oral Sex with a Man
The risk of HIV transmission through oral sex with a man is very low because the mouth is an unfriendly environment for HIV. A person receiving oral sex is generally not at risk because that person is coming into contact only with saliva, which does not transmit HIV. However, the presence of other sexually transmitted infections can increase the risk of HIV transmission during oral sex.
The minimal risk of transmission from oral sex with a man is only for the person performing the oral sex. Open cuts and abrasions in the mouth or bleeding gums can create an entry point for HIV and increase the risk of transmission. Learn how to reduce your risk of HIV transmission during oral sex with a man.
There are a few documented cases where it appears that HIV was transmitted orally and those cases are attributed to ejaculation into the mouth. Saliva contains enzymes that break down the virus and the mucous membranes in the mouth are more protective than anal or vaginal tissue.
Oral Sex with a Woman
The risk of transmission through oral sex with a woman is very low because the mouth is an unfriendly environment for HIV. Saliva breaks down the virus and the mucous membranes in the mouth are more protective than anal or vaginal tissue. The minimal risk of transmission from oral sex with a woman is only for the person performing the oral sex as their mouth is in contact with vaginal fluid. However, there is little data documenting HIV transmission via oral sex from an infected woman to an uninfected person.
Performing oral sex on a woman who is menstruating increases the risk because blood has more HIV than vaginal fluid.
A person receiving oral sex is generally not at risk because that person is coming into contact only with saliva, which does not transmit HIV. The presence of other sexually transmitted infections can increase the risk of HIV transmission during oral sex.
Oral to Anal Sex
Oral to anal contact (rimming) poses minimal risk for HIV transmission. However, rimming is a risk for transmission of hepatitis, parasites, and many other sexually transmitted infections.
HIV can be transmitted by contact between infectious fluids and bleeding cuts or open sores in the skin. However, healthy intact skin does not allow HIV to enter the body and provides an excellent barrier against the virus.
Non-sexual transmission is rare. The rare circumstances where non-sexual transmission has occurred typically involve medical settings or accident scenes where there is a very large volume of blood exposure or a needle stick.
Injection Drug Use
Sharing syringes poses a very high risk for HIV transmission. Sharing a syringe is the most efficient way to transmit the virus as it passes blood directly from one person’s blood stream to another’s. Sharing syringes is also a very efficient way to transmit other blood borne viruses such as Hepatitis B and Hepatitis C.
Tattoos and Piercings
There have been no documented cases of transmission of HIV by piercing or tattooing. However, there are documented cases of Hepatitis B transmission. Since Hepatitis B and HIV are transmitted by the same activities, there is a theoretical risk of HIV transmission through tattoos and piercing.
Mother to Infant Transmission
It is possible for a mother who has HIV to pass the virus to her baby by exposure to blood and vaginal fluids during birth or through breast milk during feeding. The risk of transmission from mother to child during pregnancy or birth can be greatly reduced by taking certain HIV medications as prescribed.
Partners Living with HIV
People living without HIV may want to consider preventive HIV medications to avoid the risk of acquiring HIV. Currently, there are two strategies for preventing HIV with antiretroviral therapy. One of the medications is taken daily, as a preventive measure. The other is taken after potential exposure to HIV.
PrEP is preventive medication for people who don’t have HIV but are at risk of acquiring it. It’s a once-daily oral medication that stops HIV from infecting cells in the immune system. The US Preventive Services Task Force (USPSTF) recommends it for everyone at increased risk of HIV.
If a person without HIV has sex with a person living with HIV who has a detectable viral load, taking PrEP can decrease their risk of acquiring HIV. PrEP is also an option if engaging in sex with a partner whose status is unknown.
The CDC states PrEP will reduce the risk of contracting HIV from sex by more than 90 percent.
A PrEP regimen involves:
- Regular medical appointments. This includes getting screened for sexually transmitted infections (STIs) and having kidney function intermittently monitored.
- Being screened for HIV. Screening takes place before getting a prescription and every three months after.
- Taking a pill each day.
PrEP may be covered by insurance. Some people might be able to find a program that subsidizes the medication. The website Please PrEP Me provides links to clinics and providers that prescribe PrEP, as well as information on insurance coverage and free or low-cost payment options.
Besides taking PrEP, also consider other options, such as using condoms. PrEP takes one to three weeks to offer protection, depending on the sexual activity. For instance, it takes longer for the medication to be effective at protecting the vagina against HIV transmission than it does the anus. Also, PrEP doesn’t protect against other STIs.
PEP is an oral medication taken after sex if there’s been a risk of exposure to HIV. This can include instances when:
- a condom breaks
- a condom wasn’t used
- someone without HIV comes in contact with blood or bodily fluids from someone with HIV and a detectable viral load
- someone without HIV comes in contact with blood or bodily fluids from someone whose HIV status is unknown to them
PEP is only effective if taken within 72 hours after exposure to HIV. It must be taken daily, or as otherwise prescribed, for 28 days.
When One Partner Is HIV Positive
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Two years after Maria Mejia, 44, and Li Laing, 46, met on social media, their deep friendship blossomed into a romance. But on the day that Mejia told Laing about her HIV status, it took her more than two hours before she could finally send a message that read, “I’m HIV positive.”
“That moment is very scary,” Mejia says. “There’s always a chance that they might .”
But to her relief, Laing’s much-anticipated response just said, “And?”
Mejia was diagnosed with HIV at the age of 18. Today, she’s an AIDS activist and educator and a blogger at The Well Project. And while Mejia has told previous partners that she has HIV, she says, the conversation doesn’t get easier with time.
Laing and Mejia, who have been together for 10 years and are now married, have maintained a strong physical and emotional bond. Their relationship, in which one partner is HIV negative and one partner is HIV positive, is called serodiscordant or mixed-status, but the preferred term among those living with HIV is “magnetic couple.” And although Laing and Mejia have their challenges — just like every couple — some issues are unique to people living with a chronic illness.
Loving Someone Who Has HIV
For Mejia and Laing, HIV can loom large over their future. Right now, Mejia is healthy — but when you’re living with HIV, she says, that could change.
“I have a lot of stamina and strength because I live a very healthy life,” Mejia says. But she’s also seen healthy, HIV-positive friends succumb to pneumonia or other serious conditions very quickly. Her partner worries that could happen to her, Mejia says, but she notes that they “live one day at a time.”
“I cannot live in the past and I cannot think, ‘What if I get sick?’” she says. “The best thing I can do is take care of myself today.”
Protecting Yourself and Your Partner
Couples in mixed-status relationships can still have an active sex life if they take some precautions. Using condoms any time you have vaginal, anal, or oral sex reduces your risk of transmitting HIV from one partner to another, according to the U.S. Department of Health and Human Services.
It’s also important to stay on top of your treatment regimen. Although antiretroviral drugs can’t cure HIV, they can significantly decrease the amount of virus in a person’s blood. In July 2017, the AIDS Prevention Access Campaign released a statement supported by the International AIDS Society that there’s evidence that an HIV-positive person with an undetectable viral load in the blood for at least six months won’t transmit the virus.
While that’s undoubtedly good news, it doesn’t mean couples can be lax about protection. Not only does viral suppression require strict adherence to an HIV treatment regimen, but also viral load may suddenly rise for various — and sometimes unpredictable — reasons, called a “blip.”
“It’s important to know your partner’s status and viral load, and that using condoms and safe injecting practices can help prevent HIV and other sexually transmitted infections,” says Antonio Urbina, MD, an associate professor of infectious disease at the Icahn School of Medicine at Mount Sinai in New York City.
HIV-negative people can also protect themselves by taking pre–exposure prophylaxis (also known as PrEP), says Shannon Weber, MSW, director of HIVE. PrEP is one pill taken once daily that is safe and highly effective in preventing HIV, Weber says.
Mejia says her viral load has been undetectable for many years, and feels comfortable that Laing isn’t at risk. When she was younger, Mejia put off taking medications, but today, she understands the importance of controlling HIV and takes them regularly.
“You’re not only saving your life, but other people’s lives as well,” Mejia says.
Getting Comfortable Disclosing Your Status
Mejia is familiar with the stigma, and often the shame, that affects people living with HIV on a daily basis. But, she says, it’s no reflection on who you are, your morals or values, or how you live your life. If you tell someone you have cancer, you get compassion — but if you tell someone that you have HIV, you often get questioned, she says.
When it comes to telling a partner about your HIV status, Weber recommends speaking up as early as possible. This way, she says, you’ll “know if this is a relationship that is worth pursuing.”
Plan ahead, practice how you’ll deliver the information, and arm yourself with answers to any potential questions. Weber also suggests bringing your partner along to one of your doctor’s visits; this way, you can ask questions together.
No matter what, don’t be discouraged. If you ever feel that no one will accept or love you because you’re living with HIV, just look to Mejia and Laing and the countless other couples living with HIV — they prove that’s simply not true.
Getting and keeping an undetectable viral load is the best thing you can do to stay healthy. Most people can get the virus under control within six months. But some people face challenges that make it hard to stick to a treatment plan, and a few people cannot get an undetectable viral load even though they take HIV medicine as prescribed.
If your viral load is not undetectable—or does not stay undetectable—you can still protect your partner by using other prevention methods. The following actions are highly effective for preventing HIV:
- Use condoms the right way every time you have anal or vaginal sex.
- Choose sexual activities with little to no risk, like oral sex. You could also use condoms or dental dams with oral sex to lower the risk even more.
- Your partner can take medicine to prevent HIV, called pre-exposure prophylaxis (PrEP). Your partner will need to take PrEP every day for around 7 days before it becomes as effective as it can be for receptive anal sex and around 20 days for receptive vaginal sex,* and they’ll need to keep taking PrEP every day.
- Never share syringes or works to inject drugs.
If your partner or you have other sexually transmitted diseases (STDs), getting treatment for those STDs can also help lower your risk of transmitting HIV. People who are HIV-positive and have another STD may have an increased concentration of HIV in their semen and genital fluids, which might make them more likely to transmit HIV. People who are HIV-negative and have another STD may have irritation that makes it easier for HIV to enter their body during sex, or inflammation in their body may increase the number of cells that HIV can target.
Keep in mind that your greatest chance of transmitting HIV is when you are the insertive partner (top) during anal sex. But it’s also possible to transmit HIV when you are the receptive partner during anal sex or either partner during vaginal sex.
* The number of days depends on the person’s HIV risk behaviors: For receptive anal sex, an HIV-negative person needs to take PrEP 7 days in a row (and keep taking it) to get the most protection. For receptive vaginal sex and injection drug use, they need to take PrEP 20 days in a row (and keep taking it) to get the most protection. We don’t know yet how long it takes to get the most protection for insertive anal or insertive vaginal sex. For more information, see page 46 in the PrEP guidelines pdf icon.
Vaginal sex and the risk of HIV transmission
- For unprotected vaginal intercourse with an HIV-positive partner with a fully suppressed viral load, the estimated risk of infection is zero.
- If HIV is not fully suppressed by effective treatment, vaginal intercourse without condoms is a high-risk route of sexual HIV transmission for both the man and the woman.
- Sexually transmitted infections increase the risk of infection while male circumcision lowers it.
If the HIV-positive partner is taking antiretroviral treatment and has a fully suppressed viral load (‘undetectable’), the risk of HIV transmission through condomless vaginal intercourse is zero.
The PARTNER study recruited 548 heterosexual couples where one partner had HIV and fully suppressed viral load on antiretroviral treatment. During a median follow-up period of 1.3 years, the couples reported over 36,000 vaginal sex acts without condoms. No HIV transmissions occurred. The investigators concluded that the risk of HIV transmission through vaginal intercourse in these circumstances was effectively zero (Rodger).
When HIV is not suppressed by antiretroviral treatment, vaginal intercourse without a condom 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 can 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, especially during adolescence and during a woman’s first pregnancy, or due to infection with human papillomavirus 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 the male partner was HIV negative. The risk was 0.08% when the male partner was HIV positive and the female partner HIV negative. However, these rates were considerably higher when the source partner was in either the very early or the late stage of HIV infection, when one partner had a sexually transmitted infection that causes genital sores, and also in studies done in lower-income countries (Boily).
A woman is usually at greater risk of HIV infection from an HIV-positive man than vice versa. This may be due to several factors. Compared with the penis, the amounts of tissue cells susceptible to infection and/or damage are likely higher within the vagina. A woman may often take large quantities of semen into her vagina, which quickly comes into contact with the more vulnerable tissue of the cervix and may remain there for a period of time.
While women are at greater risk of infection from an HIV-positive male partner, condomless 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.
Many other factors affect the level of risk associated with vaginal intercourse, including recent infection, sexually transmitted infections and male circumcision.
The first few weeks or months after HIV has entered a person’s body, their viral load is usually extremely high and they are very infectious.
Viral load is the term used to describe the amount of HIV circulating in the body. As viral load rises, so does infectiousness. On the other hand, when viral load is so low as to be undetectable, there is no risk at all of HIV transmission.
sexually transmitted infections (STIs)
Although HIV can be sexually transmitted, the term is most often used to refer to chlamydia, gonorrhoea, syphilis, herpes, scabies, trichomonas vaginalis, etc.
voluntary male medical circumcision (VMMC)
The surgical removal of the foreskin of the penis (the retractable fold of tissue that covers the head of the penis) to reduce the risk of HIV infection in men.
Measurement of the amount of virus in a blood sample, reported as number of HIV RNA copies per milliliter of blood plasma. The VL is an important indicator of HIV progression and of how well treatment is working.
The surgical removal of the foreskin of the penis (the retractable fold of tissue that covers the head of the penis) to reduce the risk of HIV infection in men.
herpes simplex virus (HSV)
A viral infection which may cause sores around the mouth or genitals.
For example, a study of heterosexual couples in Rakai, Uganda, where one partner was HIV positive and the other HIV negative at the start of the study, showed that the likelihood of HIV transmission is highest in the first two and a half months following initial infection with HIV, and that this correlated with higher viral load levels in early HIV infection. The researchers estimated that relative to chronic infection, infectiousness during primary infection was enhanced 26-fold (Hollingsworth).
As people are usually unaware of their infection at this stage, they are not taking treatment and may inadvertently expose sexual partners to HIV. People who have HIV without realising it are much more likely to be involved in HIV transmission than people who know they have HIV, as the latter can receive treatment.
Sexually transmitted infections
Most sexually transmitted infections, especially those which cause ulceration, including herpes simplex 2 (HSV-2), syphilis, gonorrhoea, and chlamydia, increase HIV-negative partners’ risk of acquiring HIV during vaginal intercourse. HIV-negative people with recurrent STIs may be at increased ongoing risk of HIV infection.
In people living with HIV who are not taking HIV treatment, STIs also lead to increased genital shedding of HIV, which increases the risk of transmitting HIV to others. Nonetheless, STIs do not increase the risk of HIV transmission from people who are taking effective HIV treatment and have an undetectable viral load. In the PARTNER study, there was not a single HIV transmission even though many study participants had STIs.
There are two main reasons for the effect of STIs on HIV transmission. Firstly, many (although not all) STIs can cause ulcers, sores or lesions. They provide a direct physical route of entry for HIV in an uninfected person. Secondly, immune cells that are, themselves, prone to HIV infection – such as activated T-cells and dendritic cells – are prone to be present in greater numbers at the site of an infection.
The strongest evidence is for herpes simplex virus type 2 (HSV-2). A meta-analysis found that having HSV-2 was associated with an increased risk of acquiring HIV both in the general population (an increase of 270%) and in higher risk groups such as sex workers and men who have sex with men (170%). Having a recent HSV-2 infection was associated with an almost five-fold (470%) increased risk of acquiring HIV, probably because genital ulceration, viral shedding and inflammation in the genital tract are most severe in new HSV-2 infections and tend to decrease over time (Looker).
Gonorrhoea, chlamydia and trichomoniasis have been shown to increase risk of HIV infection in women (Laga). Trichomonas vaginalis increases the risk of HIV acquisition for women (Masha).
Human papillomavirus, the cause of genital warts, is associated with an increased risk of HIV infection in women regardless of whether it is an HPV type that causes genital warts or a type associated with cervical cancer (Houlihan). The presence of human papillomavirus in cells in the penis also increases the risk of acquisition in men (Rositch).
Men are less likely to acquire HIV through vaginal intercourse if they are circumcised. There is strong biological and epidemiological evidence for this.
Circumcision is believed to reduce the risk of male infection because it removes the vulnerable tissue inside the foreskin, which contains Langerhans cells (a type of cell particularly vulnerable to HIV infection). The area under the foreskin is also vulnerable to trauma, and is more likely to become abraded if sufficient lubrication is not present. Also, uncircumcised men may be more vulnerable to sexually transmitted infections (STIs), because the area under the foreskin can retain bacteria acquired during sex, thus increasing the chance that an infection will become established.
Three randomised controlled trials of circumcision demonstrated that circumcision lowers HIV-negative men’s risk of acquiring HIV by between 51 and 60%. They also showed benefits in terms of lowering rates of sexually transmitted infections. The studies were conducted in South Africa (Auvert), Kenya (Bailey) and Uganda (Gray).
The World Health Organization recommends that circumcision programmes should be an integral part of HIV prevention programmes in countries with generalised HIV epidemics. In the United States, the Centers for Disease Control and Prevention (CDC) recommends that healthcare providers can inform their patients that medical male circumcision “male circumcision reduces, but does not eliminate, the risk of acquiring HIV” by men during vaginal sex.
Male circumcision does not reduce the risk of HIV transmission from men to female partners.
Schistosomiasis (also known as bilharzia) is a widespread infection in sub-Saharan Africa and other tropical countries. Schistosomiasis is caused by a parasitic worm that lives in fresh water and is acquired by bathing in infested water. The infection can cause a localised immune response and genital lesions, increasing the risk of HIV transmission and acquisition.
In a study of over 1000 HIV-serodiscordant couples in Zambia (Wall), women who had schistosomiasis had an 40% increased risk of acquiring HIV. In addition, when an HIV-positive man or woman had schistosomiasis, there was a greater risk of HIV transmission to their sexual partner.
Bacterial vaginosis is a type of vaginal inflammation caused by the overgrowth of bacteria naturally found in the vagina, which upsets the natural balance. Signs and symptoms may include a discharge, an odour, itching and burning during urination.
It appears that bacterial vaginosis is associated with an increased risk of acquiring HIV. A meta-analysis of four prospective studies conducted in sub-Saharan Africa found a 61% increased risk of HIV acquisition in women (Atashhili). In a Kenyan study, bacterial vaginosis and HSV-2 infection were the two strongest risk factors measured for HIV acquisition over a 20-year period (Masese).
Practices such as douching, washing and drying the vagina may undermine the body’s innate defences and make bacterial vaginosis more common.
Withdrawal before ejaculation
The chances of infection may be lessened during condomless vaginal intercourse if ejaculation does not take place. An early study found that after 20 months, none of the heterosexual couples who had consistently practised withdrawal experienced the seroconversion of the HIV-negative partner (De Vincenzi).
Rougher sex and/or vaginal dryness may lead to vaginal or penile abrasions, which may increase the chances of infection.
Reduction of vaginal lubrication becomes more common with age, possibly increasing risk in post-menopausal women. Younger women in early puberty may also produce less vaginal and cervical secretions, perhaps increasing their vulnerability to HIV infection and contributing to the disproportionate prevalence of HIV amongst adolescent women (Holmberg).
How you can reduce the risk
Effective protective measures are:
- The HIV-positive partner taking effective HIV treatment and having an undetectable viral load.
- The HIV-negative partner taking pre-exposure prophylaxis (PrEP).
- Condoms and lubricant.
- As an emergency measure, post-exposure prophylaxis (PEP).
Service providers working in HIV prevention are often asked by their patients and clients about the risk of HIV transmission from an exposure to HIV through sex. What do the latest studies tell us about this risk? And how should we interpret and communicate the results?
Challenges in calculating a number
It isn’t easy for researchers to calculate the risk of transmission from an exposure to HIV through sex. To do this effectively, a group of HIV-negative individuals need to be followed over time and their exposures to HIV—both the number of times they are exposed and the types of exposure—need to be tracked.
As you can imagine, accurately tracking the number of times a person is exposed to HIV is very difficult. Researchers ask HIV-negative individuals enrolled in these studies to report how many times they have had sex in a given period of time, what type of sex they had, how often they used condoms and the HIV status of their partner(s). Because a person may have trouble remembering their sexual behaviour or may not want to tell the whole truth, this reporting is often inaccurate.
Furthermore, a person does not always know the HIV status of their partner(s). For this reason, researchers usually enroll HIV-negative individuals who are in stable relationships with an HIV-positive partner (also known as serodiscordant couples). Researchers can then conclude that any unprotected sex reported by a study participant counts as an exposure to HIV.
Several studies have aimed to estimate the average risk of HIV transmission from a specific type of unprotected sex (for example, vaginal/anal/oral; insertive/receptive). Due to the difficulties of calculating this risk, these studies have produced a wide range of numbers. To come up with a more accurate estimate for each type of unprotected sex, some researchers have combined the results of individual studies into what is known as a meta-analysis.
All exposures are not equal
The results of several meta-analyses suggest that some types of sex carry on average a higher risk of HIV transmission than others. Below are estimates from meta-analyses that have combined the results of studies conducted in high-income countries. For types of sex where meta-analysis estimates do not exist, numbers from individual studies are provided.
UPDATE: New HIV risk estimates by exposure are available.
A meta-analysis exploring the risk of HIV transmission through unprotected anal sex was published in 2010.1 The analysis, based on the results of four studies, estimated the risk through receptive anal sex (receiving the penis into the anus, also known as bottoming) to be 1.4%. (This means that an average of one transmission occurred for every 71 exposures.) This risk was similar regardless of whether the receptive partner was a man or woman.
No meta-analysis estimates currently exist for insertive anal sex (inserting the penis into the anus, also known as topping) but two individual studies were conducted to calculate this risk. The first, published in 1999, calculated the risk to be 0.06% (equivalent to one transmission per 1,667 exposures).2 However, due to the design of the study, this number likely underestimated the risk of HIV transmission. The second study, published in 2010, was better designed and estimated the risk to be 0.11% (or 1 transmission per 909 exposures) for circumcised men and 0.62% (1 transmission per 161 exposures) for uncircumcised men.3
A meta-analysis of 10 studies exploring the risk of transmission through vaginal sex was published in 2009.4 It estimated the risk of HIV transmission through receptive vaginal sex (receiving the penis in the vagina) to be 0.08% (equivalent to 1 transmission per 1,250 exposures).
A meta-analysis of three studies exploring the risk from insertive vaginal sex (inserting the penis into the vagina) was estimated to be 0.04% (equivalent to 1 transmission per 2,500 exposures).4
No meta-analysis estimates exist for oral sex (vaginal or penile) because too few good-quality studies have been completed. This is because it is difficult to find people whose only risk of HIV transmission is unprotected oral sex. A review of the studies that are available was published in 2008 and concluded that vaginal and penile oral sex pose a “low but non-zero transmission probability.”5
In the three studies aimed at calculating the risk of HIV transmission from one act of oral sex, no transmissions were observed among three different populations—lesbian serodiscordant couples, heterosexual serodiscordant couples and single gay men—who reported unprotected oral sex as their only risk for HIV transmission. However, these studies enrolled only a small number of people and followed them for only a short period of time, which may explain the lack of HIV transmissions and makes it impossible to conclude that the risk from oral sex is zero.
Interpreting the numbers—what additional information needs to be provided?
Some clients may see these numbers and think their risk of HIV transmission is low. Therefore, caution is needed when interpreting them. If these numbers are provided to clients, they should be accompanied by information that helps shed light on why the risk may be higher than it seems.
Transmission can occur after one exposure.
It is important to emphasize that a person could become infected from having unprotected sex once or a person could have unprotected sex many times and not become infected, regardless of how low or high the risk per exposure is.
A risk of 1% would mean that an average of one infection would occur if 100 HIV-negative people were exposed to HIV through a certain type of sex. It does not mean that a person needs to be exposed 100 times for HIV infection to occur.
These are estimates of average risk in the absence of biological factors that increase risk.
The numbers in the table above are rough estimates. They are averages and do not represent the risk from all exposures to HIV through a certain type of sex.
We know that no two exposures to HIV are exactly the same. Research shows that, in addition to the type of sex that led to the exposure, several factors can increase or decrease the risk that an exposure to HIV leads to infection. These include the presence of sexually transmitted infections (STIs), a high viral load, a man being uncircumcised, a woman menstruating, other bleeding and activities that can cause tearing and inflammation, such as rough sex, longer sex, douching, enemas before anal sex, and tooth brushing, flossing or dental work before oral sex. Each exposure to HIV carries a unique risk of transmission that depends on the type of sex and a combination of biological factors.
The risk of HIV transmission may be much higher than these averages if biological risk factors are present. For example, research shows that STIs and some vaginal conditions, such as bacterial vaginosis, can increase the risk of HIV transmission by up to 8 times.6,7,8 As a result, the risk of an HIV-negative woman becoming infected through unprotected receptive vaginal sex could be closer to 1% (1 transmission per 100 exposures) if she has a vaginal STI.
We also know that for every 10-fold increase in viral load, the risk of HIV transmission increases by 2 to 3 times.9,10 Research suggests the extremely high viral load during acute HIV infection (the first few weeks after becoming infected with HIV) can increase the risk of HIV transmission by up to 26-fold.11,12 Therefore, unprotected sex with an HIV-positive person who has acute HIV infection could carry a transmission risk of up to 2% (the equivalent of 1 transmission per 50 exposures) for receptive vaginal sex and over 20% (equivalent to 1 transmission per 5 exposures) for receptive anal sex.
The more exposures, the greater the risk.
Although the risk of HIV transmission from a single exposure may seem low to some people, this risk increases over multiple exposures. In other words, a person who is exposed to HIV more often has a greater overall risk of HIV transmission than someone who is exposed less often.
If a woman has unprotected vaginal sex 100 times with a man who is HIV-positive, the cumulative risk is approximately 10% and may be higher if biological risk factors are present.
Differences in risk
Information on how risky certain types of unprotected sex are compared to others may help people make more informed decisions about the type of sex they are having.
Based on the meta-analysis estimates, we can draw several conclusions:
- Receptive anal sex carries a much higher risk of HIV infection than receptive vaginal sex.
Research shows that the risk of HIV transmission from receptive anal sex is up to 18 times higher than from receptive vaginal sex.
- Receptive anal sex is riskier than insertive anal sex.
Research suggests the risk of HIV transmission from receptive anal sex is 3 to 23 times higher than from insertive anal sex.
- Receptive vaginal sex is riskier than insertive vaginal sex.
The risk from receptive vaginal sex is about twice as high as that from insertive vaginal sex.
- It is unclear exactly how much less risky oral sex is compared to vaginal and anal sex.
Although it’s impossible to provide a client with their exact risk of HIV transmission from an exposure, some studies have managed to estimate an average risk for different types of sex. It’s important to provide clients with additional information to help them interpret the findings. Here are some key messages:
- These numbers
- are challenging to calculate and should therefore be considered rough estimates
- do not represent the risk of transmission from all exposures to HIV
- represent the average risk of transmission in the absence of biological factors that can increase risk (such as STIs and a high viral load)
- are most relevant to people in stable monogamous serodiscordant relationships
- These numbers may seem low but
- HIV transmission can occur after a single exposure
- the risk may be much higher if certain biological risk factors, such as STIs or a high viral load, are present
- as more exposures to HIV occur, the overall risk of transmission increases
- most HIV transmissions in Canada occur through unprotected anal and vaginal sex
- There are several ways of reducing the risk of HIV transmission from an exposure, such as post-exposure prophylaxis (PEP), using antiretroviral treatment to reduce viral load, circumcision, treatment for STIs and vaginal conditions, or engaging in lower-risk activities.
- There is no way to reduce the risk of HIV transmission to zero after an exposure occurs. Taking measures to avoid an exposure in the first place (for example, through the correct use of condoms or other barrier methods, or by ensuring a partner has the same HIV status) can help reduce the overall risk of HIV transmission.
- 1. Baggaley RF, White RG, Boily M-C. HIV transmission risk through anal intercourse: systematic review, meta-analysis and implications for HIV prevention. International Journal of Epidemiology. 2010 Aug;39(4):1048–63.
- 2. Vittinghoff E, Douglas J, Judson F et al. Per-contact risk of human immunodeficiency virus transmission between male sexual partners. American Journal of Epidemiology. 1999 Aug 1;150(3):306–11.
- 3. Jin F, Jansson J, Law M et al. Per-contact probability of HIV transmission in homosexual men in Sydney in the era of HAART. AIDS. 2010 Mar 27;24(6):907–13.
- 4. a. b. Boily M-C, Baggaley RF, Wang L et al. Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies. Lancet Infectious Diseases. 2009 Feb;9(2):118–29.
- 5. Baggaley RF, White RG, Boily M-C. Systematic review of orogenital HIV-1 transmission probabilities. International Journal of Epidemiology. 2008 Dec;37(6):1255–65.
- 6. Ward H, Rönn M. Contribution of sexually transmitted infections to the sexual transmission of HIV. Current Opinion in HIV and AIDS. 2010 Jul;5(4):305–10.
- 7. Atashili J, Poole C, Ndumbe PM et al. Bacterial vaginosis and HIV acquisition: a meta-analysis of published studies. AIDS. 2008 Jul 31;22(12):1493–501.
- 8. Cohen CR, Lingappa JR, Baeten JM et al. Bacterial vaginosis associated with increased risk of female-to-male HIV-1 transmission: a prospective cohort analysis among African couples. PLoS Medicine. 2012 Jun;9(6):e1001251.
- 9. Wawer MJ, Gray RH, Sewankambo NK et al. Rates of HIV-1 transmission per coital act, by stage of HIV-1 infection, in Rakai, Uganda. Journal of Infectious Diseases. 2005 May 1;191(9):1403–9.
- 10. Baeten JM, Kahle E, Lingappa JR et al. Genital HIV-1 RNA predicts risk of heterosexual HIV-1 transmission. Science Translational Medicine. 2011 Apr 6;3(77):77ra29.
- 11. Wawer MJ, Gray RH, Sewankambo NK et al. Rates of HIV-1 transmission per coital act, by stage of HIV-1 infection, in Rakai, Uganda. Journal of Infectious Diseases. 2005 May 1;191(9):1403–9.
- 12. Hollingsworth TD, Anderson RM, Fraser C. HIV-1 transmission, by stage of infection. Journal of Infectious Diseases. 2008 Sep 1;198(5):687–93.
There are several ways to prevent getting or transmitting HIV through sex.
If you are HIV negative, you can use HIV prevention medications known as pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP) to protect yourself. You can also use other HIV prevention methods, below.
If you are living with HIV, the most important thing you can do to prevent transmission and stay healthy is to take your HIV medication (known as antiretroviral therapy or ART), every day, exactly as prescribed. People living with HIV who take HIV medication daily as prescribed and get and keep an undetectable viral load have effectively no risk of sexually transmitting HIV to their HIV-negative partners. There also are other options to choose from, below.
How Can You Prevent Getting HIV from Anal or Vaginal Sex?
If you are HIV-negative, you have several options for protecting yourself from HIV. The more of these actions you take, the safer you can be.
- Use condoms. Condoms are highly effective at preventing HIV infection if you use them the right way every time you have sex. Learn the right way to use a male condom and a female condom.
- Reduce your number of sexual partners. This can lower your chances of having a partner who could transmit HIV to you. The more partners you have, the more likely you are to have a partner with HIV whose viral load is not suppressed or to have a sex partner with a sexually transmitted disease. Both of these factors can increase the risk of HIV transmission.
- Talk to your doctor about PrEP. Pre-exposure prophylaxis (PrEP) is daily medication that can reduce your chance of getting HIV. Taken every day, PrEP can stop HIV from taking hold and spreading throughout your body. PrEP might benefit you if you are HIV-negative and have an ongoing relationship with an HIV-positive partner PrEP also should be considered if you aren’t in a mutually monogamous relationship with a partner who recently tested HIV-negative, and you are a:
- gay or bisexual man who has had anal sex without a condom or been diagnosed with an STD in the past 6 months;
- man who has sex with both men and women; or
- heterosexual man or woman who does not regularly use condoms during sex with partners of unknown HIV status who are at very high risk of HIV infection (for example, people who inject drugs or women who have bisexual male partners).
- Take PEP within 72 hours after a possible HIV exposure. Post-exposure prophylaxis (PEP) means taking HIV medication after being potentially exposed to HIV to prevent becoming infected. If you’re HIV-negative or don’t know your HIV status and think you have recently been exposed to HIV during sex (for example, if the condom breaks), talk to your health care provider or an emergency room doctor about PEP right away (within 3 days). The sooner you start PEP, the better; every hour counts. If you’re prescribed PEP, you’ll need to take it once or twice daily for 28 days. Keep in mind that your chance of getting HIV is lower if your HIV-positive partner is taking HIV medication daily prescribed and his or her viral load is undetectable.
- Get tested and treated for other STDs and encourage your partners to do the same. If you are sexually active, get tested at least once a year. Having other STDs increases your risk for getting or transmitting HIV. STDs can also have long-term health consequences. .
- If you’re HIV-negative and your partner is HIV-positive, encourage your partner to get and stay on HIV treatment. If taken daily as prescribed, HIV medication (ART) reduces the amount of HIV in the blood (the viral load) to a very low levels—so low that a standard lab test can’t detect it. This is called having an undetectable viral load. People living with HIV who take HIV medication daily as prescribed and get and keep an undetectable viral load can stay healthy and have effectively no risk of sexually transmitting HIV to an HIV-negative partner through sex.
- Choose less risky sexual behaviors. HIV is mainly spread by having anal or vaginal sex without a condom or without taking medicines to prevent or treat HIV. Here is some information about the risk associated with specific sexual behaviors. See CDC’s HIV Risk Reduction Tool (BETA) for more information.
- Receptive anal sex is the riskiest type of sex for getting HIV. It’s possible for either partner—the partner inserting the penis in the anus (the top) or the partner receiving the penis (the bottom)—to get HIV, but it is much riskier for an HIV-negative partner to be the receptive partner. That’s because the lining of the rectum is thin and may allow HIV to enter the body during anal sex.
- Vaginal sex also carries a risk for getting HIV, though it is less risky than receptive anal sex. Most women who get HIV get it from vaginal sex, but men can also get HIV from vaginal sex.
- In general, there is little to no risk of getting or transmitting HIV from oral sex. Theoretically, transmission of HIV is possible if an HIV-positive man ejaculates in his partner’s mouth during oral sex. However, the risk is still very low, and much lower than with anal or vaginal sex. Factors that may increase the risk of transmitting HIV through oral sex are oral ulcers, bleeding gums, genital sores, and the presence of other STDs, which may or may not be visible. For more information, see CDC’s HIV Basics: How can I prevent getting HIV from oral sex?
- Sexual activities that don’t involve contact with body fluids (semen, vaginal fluid, or blood) carry no risk of HIV transmission but may pose a risk for other STDs.
Is Abstinence an Effective Way to Prevent HIV?
Yes. Abstinence means not having oral, vaginal, or anal sex. An abstinent person is someone who’s never had sex or someone who’s had sex but has decided not to continue having sex for some period of time. Abstinence is the only 100% effective way to prevent HIV, other sexually transmitted diseases (STDs), and pregnancy. The longer you wait to start having oral, vaginal, or anal sex, the fewer sexual partners you are likely to have in your lifetime. Having fewer partners lowers your chances of having sex with someone who has HIV or another STD.
If You Are Living with HIV, How Can You Prevent Passing It to Others?
If you are living with HIV, there are many actions you can take to prevent transmitting HIV to an HIV-negative partner. The more of these actions you take, the safer you can be.
- Take HIV medication. The most important thing you can do is to take medication to treat HIV infection (called antiretroviral therapy, or ART) every day, exactly as prescribed. Taking HIV medication daily as prescribed can make the amount of HIV in your blood (your viral load) very low—so low that a standard lab test can’t detect it. This is called having an undetectable viral load. Getting and keeping an undetectable viral load is the best thing you can do to stay healthy. If your viral load stays undetectable, you have effectively no risk of transmitting HIV to an HIV-negative partner through sex. Learn more about HIV treatment as prevention.
- If you’re taking HIV medication, follow your health care provider’s advice. Visit your health care provider regularly and always take your medication as prescribed.
- Use condoms the right way every time you have sex. Learn the right way to use a male condom and a female condom.
- Choose less risky sexual behaviors. Anal sex is the highest-risk sexual activity for HIV transmission. If your partner is HIV-negative, it’s less risky if they’re the insertive partner (top) and you’re the receptive partner (bottom) during anal sex. Oral sex is much less risky than anal or vaginal sex. Sexual activities that don’t involve contact with body fluids (semen, vaginal fluid, or blood) carry no risk of HIV transmission.
- If you inject drugs, never share your needles or works with anyone.
- Talk to your HIV-negative partners about PrEP. Pre-exposure prophylaxis (PrEP) is daily medication that can reduce an HIV-negative person’s chance of getting HIV. Taken every day, PrEP can stop HIV from taking hold and spreading throughout throughout the person’s body.
- Talk to your HIV-negative partners about post-exposure prophylaxis (PEP) if you think they’ve recently had a possible exposure to HIV (for example, if they had anal or vaginal sex without a condom or if the condom broke during sex). Your partners should talk to a health care provider right away (within 72 hours) after a possible exposure. Starting PEP immediately and taking it daily for 28 days will reduce their chance of getting HIV.
- Get tested and treated for other STDs and encourage your partners to do the same. If you are sexually active, get tested at least once a year. STDs can have long-term health consequences. They can also increase the risk of getting or transmitting HIV. .
Also, encourage your partners who are HIV-negative to get tested for HIV so they are sure about their status and can take action to keep themselves healthy. Use HIV.gov’s HIV Testing Sites & Care Services Locator to find a testing site nearby.
Learn more by visiting CDC’s HIV Prevention Basics. You can also get information on how to protect yourself and your partner that is tailored to meet your needs from CDC’s HIV Risk Reduction Tool (BETA).