How can you evaluate different levels of risk? Scientists continue to investigate this question, among them Omar Harfouch, MD, MPH, a physician and researcher who specializes in HIV treatment and prevention at the Institute of Human Virology at the University of Maryland School of Medicine in Baltimore. Here Dr. Harfouch explains the different ways people can get HIV, provides stats about the relative risk of each behavior, and discusses measures people can take that can dramatically lower the odds of transmission.
Odds of Getting HIV From Anal Sex
Receptive anal sex Receptive anal intercourse without a condom poses the highest HIV transmission risk of any sexual act. Harfouch says the odds of transmitting HIV this way are between .5 and 1.38 percent. The receptive (“bottom”) partner is 13 times more likely to get HIV than the insertive (“top”) partner, according to the Centers for Disease Control and Prevention (CDC). That’s because HIV — found in blood, semen, pre-seminal fluid (“pre-cum”), and rectal fluids — can cross the thin lining of the rectum with relative ease, per the CDC. Insertive anal sex The insertive (“top”) partner is less likely than the receptive partner to get HIV from someone with HIV: The risk is between .07 and .1 percent, says Harfouch. Bodily fluids carrying the virus can enter the insertive person through the urethra (the opening at the tip of the penis) or any cuts or sores on the penis.
Odds of Getting HIV From Vaginal Sex
Receptive vaginal sex Close to one in five new HIV diagnoses in the United States are women, and the most common way that women get HIV is through sex with a male partner who has HIV. Receptive vaginal intercourse has about a .1 percent risk, says Harfouch. A man with HIV can transmit the virus to a female partner through pre-seminal fluid or semen, which allows HIV to pass through the lining of the vagina and cervix. Insertive vaginal sex A woman living with HIV can transmit the virus to a male partner through vaginal fluid and blood, which may pass through the urethra (the opening at the tip of the penis), the foreskin (if the man isn’t circumcised), or any open sores on the penis. The risk of transmitting HIV through insertive vaginal intercourse is between .05 to .04 percent, says Harfouch.
Odds of Getting HIV From Oral Sex
The odds of getting HIV during oral sex are slim to none. That’s because the virus isn’t transmitted through saliva (or tears or sweat) unless it’s also mixed with blood. But if a man who has HIV ejaculates in a partner’s mouth — and, for example, the partner has an open sore or bleeding gums — experts say it is theoretically possible to pass along the virus. People can reduce this hypothetical risk by having their male partner ejaculate into a condom or by removing their mouth from the penis before ejaculation.
Odds of Getting HIV From Shared Needles
People who inject drugs are at high risk of getting HIV if they use and share needles, syringes, or other drug injection equipment (for example, cookers) that someone with HIV has also used, according to the CDC. The risk of acquiring HIV this way is estimated to be .6 percent, says Harfouch. This number comes from a meta-analysis published in the journal AIDS that is one of the two main studies that have looked at per-act risk. About 10 percent of HIV diagnoses can be attributed to the use of injection drugs, according to the CDC. The reason is that needles, syringes, and other equipment can contain blood, and therefore HIV, and directly transmit the virus into another person’s bloodstream. Under the right environmental circumstances, the virus can survive in a used needle up to 42 days. In addition, drug use can lower inhibitions, making people less likely to use a condom during sex or to take medications that can prevent HIV from taking hold after an exposure, further increasing the risk of transmission. The number of HIV diagnoses among people who inject drugs has been increasing since 2014, a trend that experts attribute to the opioid and heroin crisis. The stimulant methamphetamine, which can be injected as well as snorted, smoked, or taken orally, also has a strong association with HIV: A study published in the Journal of Acquired Immune Deficiency Syndrome in November 2020 estimated that people who regularly use meth account for about one in three new HIV transmissions among sexual and gender minorities.
What to Do if You Are Exposed to HIV
If you don’t have HIV but believe you may have been exposed to the virus, you can take an emergency medication called post-exposure prophylaxis, or PEP, which can lower your odds of infection. For the medication to work as effectively as possible, you will want to start it within 72 hours of exposure — the sooner, the better — and take it as prescribed for 28 days.
How to Lower Your Risk of Getting or Spreading HIV
Pre-exposure medication If you’re at a very high risk of infection — for example, if your current sexual partner has HIV — taking an oral or injectable medication called pre-exposure prophylaxis, or PrEP, can lower your risk of HIV from sex by about 99 percent. Oral PrEP medication can also cut the risk of transmission among injection drug users by more than 74 percent. If you have HIV, you can drastically reduce the odds of passing along the virus by taking antiretroviral therapy, or ART. These medications can lower the amount of HIV that resides in the body (called viral load) to a level that’s undetectable by modern testing. A person with an undetectable viral load cannot transmit HIV via vaginal, anal, or oral sex. This has been confirmed by large research studies that have monitored thousands of male-female and male-male couples in which one partner has HIV, according to HIV.gov. “Undetectable is untransmittable,” says Harfouch. “If someone who is known to have HIV and is taking their medication as prescribed has an undetectable HIV viral load, their risk of transmitting the virus is nonexistent.” Condoms Condoms are another effective way to prevent the spread of HIV. Male condoms are estimated to be 90 percent effective in reducing HIV transmission, according to USAID, and female condoms, also known as dental dams (thin pieces of latex that can be placed over the vagina or anus), can reduce HIV transmission risk by 94 percent. Water- or silicone-based lubricants can help lessen the chance that the condom will break. Condoms can also lower the risk of transmitting not only HIV but other sexually transmitted diseases, including syphilis, herpes, gonorrhea, and chlamydia. Sterile needles and syringes In addition to taking PrEP, people who inject drugs can lower their HIV risk by only using sterile needles and syringes. These can be obtained without a prescription at pharmacies and through syringe services programs at state or local health departments. To find substance abuse help, call the Substance Abuse and Mental Health Services Administration’s national helpline at 800-662-HELP (4357) or visit its website for a list of treatment facilities near you.
The Bottom Line: You Can Protect Yourself Against HIV
No matter how you assess the odds of a potential HIV exposure leading to infection, take steps to protect yourself, says Harfouch. “If you think you might be at risk for HIV, talk to your healthcare provider,” says Harfouch. If you don’t have access to a doctor, go to your local health department.
