What Is the Risk of Contracting HIV?

HIV transmission risk varies significantly depending on the type of exposure. According to data from the U.S. Centers for Disease Control and Prevention (CDC) and peer-reviewed epidemiological studies, the per-act probability of HIV acquisition differs markedly across transmission routes.

For receptive anal intercourse—the highest-risk sexual activity—the estimated risk is approximately 1.38% per exposure when the partner is living with untreated HIV. For insertive anal intercourse, the risk drops to about 0.11%. Vaginal intercourse carries a lower per-act risk: roughly 0.08% for receptive partners and 0.04% for insertive partners.

Sharing needles or syringes for injection drug use poses a substantial risk—about 0.63% per act—due to direct blood-to-blood contact. In contrast, oral sex carries an extremely low, though not zero, theoretical risk; no confirmed cases of HIV transmission via oral sex alone have been documented in large-scale epidemiological studies.

It’s critical to emphasize that these figures reflect average risks under specific conditions—such as absence of antiretroviral therapy (ART), pre-exposure prophylaxis (PrEP), or post-exposure prophylaxis (PEP)—and do not account for individual variables like viral load, presence of other sexually transmitted infections, or mucosal integrity. When a person living with HIV achieves and maintains viral suppression through effective ART, the risk of sexual transmission is effectively zero—a concept known as “Undetectable = Untransmittable” (U=U), supported by robust clinical evidence including the PARTNER and Opposites Attract studies.

Preventive interventions—including consistent condom use, daily oral PrEP (e.g., tenofovir/emtricitabine), event-driven PrEP for men who have sex with men, and prompt initiation of PEP within 72 hours of potential exposure—can reduce transmission risk by more than 99% when used correctly and consistently.