What is the risk of Human Immunodeficiency Virus (HIV) transmission from a female to a male per act of unprotected sex?

HIV Transmission Risk: Female-to-Male Per Unprotected Vaginal Intercourse

The per-act risk of HIV transmission from an HIV-positive female to an HIV-negative male through unprotected vaginal intercourse is approximately 1 in 700 to 1 in 3,000 exposures (0.03-0.14%). 1

Baseline Transmission Probability

The insertive vaginal intercourse (female-to-male) transmission risk is substantially lower than receptive exposures:

• Female-to-male vaginal intercourse: 1 in 700 to 1 in 3,000 per act 1
• For comparison, male-to-female vaginal intercourse carries higher risk at 1 in 200 to 1 in 2,000 (0.1-0.2%) 1
• Research studies estimate male-to-female transmission at 0.0005-0.0026 per coital act, with female-to-male being even lower 2, 3

Critical Risk Modifiers That Dramatically Alter Transmission Probability

Your actual risk depends heavily on these factors, which can increase transmission probability by 10-50 fold:

Factors That Increase Risk:

• High viral load in the HIV-positive partner is the single most important risk amplifier 4, 1
• Presence of sexually transmitted infections (STIs) in either partner dramatically increases transmission through mucosal inflammation and increased viral shedding 4, 1
• Genital ulcerative diseases (herpes, syphilis, chancroid) create entry points for the virus 4
• Trauma or bleeding during intercourse substantially elevates risk 1
• Lack of male circumcision increases the receptive partner's risk 1
• Recent HIV infection in the source partner (acute/primary infection phase) when viral loads are extremely high 4

Factors That Decrease Risk:

• Antiretroviral therapy with viral suppression in the HIV-positive partner reduces transmission risk by approximately 96% 1
• Pre-exposure prophylaxis (PrEP) in the HIV-negative partner provides substantial protection 1
• Consistent condom use combined with antiretroviral treatment reduces risk by 99.2% 5

Post-Exposure Management Algorithm

If you have had a potential exposure within the last 72 hours, follow this protocol:

Immediate Action (Within 72 Hours, Ideally 24 Hours):

1. Seek medical evaluation immediately - do not wait 4, 6
2. Post-exposure prophylaxis (PEP) should be initiated using a 28-day course of combination antiretroviral therapy 4, 1, 6
3. PEP effectiveness decreases dramatically after 72 hours and is unlikely to provide benefit if started later 4, 6

When to Start PEP:

• Start immediately if the source partner is known to be HIV-positive 4
• Consider starting if the source is from a high HIV prevalence group (injection drug users, men who have sex with men, commercial sex workers) even if their status is unknown 4
• Can be started pending HIV testing of the source and discontinued if they test negative 4

Testing Protocol:

• Baseline rapid HIV test before starting PEP (do not delay first dose for results) 6
• Follow-up testing at 4-6 weeks and 12 weeks post-exposure 6

Prevention Strategies for Ongoing Risk

If you have frequent exposures rather than a single incident:

• Do NOT use repeated courses of PEP - this is inappropriate for frequent exposures 4
• Pre-exposure prophylaxis (PrEP) should be offered immediately for ongoing high-risk sexual practices 1, 7
• Comprehensive STI screening every 3-6 months at all exposure sites is essential 1, 7
• Partner with known HIV should achieve viral suppression through antiretroviral therapy, which reduces transmission by 96% 1

Common Pitfalls to Avoid

• Do not underestimate the impact of STIs - their presence can increase transmission risk by 10-50 fold 4, 1
• Do not delay PEP beyond 72 hours - effectiveness drops precipitously 4, 6
• Do not stop PEP prematurely - the full 28-day course is essential 6
• Do not assume low baseline risk means no risk - individual exposures vary enormously based on viral load and cofactors 2, 3
• Do not use PEP as a substitute for ongoing prevention if exposures are recurrent 4