What are the per‑act HIV transmission risks for unprotected vaginal intercourse, receptive and insertive anal intercourse, needle sharing, blood transfusion, and mother‑to‑child transmission, and what preventive measures are recommended?

HIV Transmission Risk: Per-Act Probabilities and Prevention

Blood transfusion carries the highest HIV transmission risk at 95%, followed by mother-to-child transmission at 25% without treatment, needle sharing at 0.67%, receptive anal intercourse at 0.5-10%, receptive vaginal intercourse at 0.05-0.5%, and insertive vaginal intercourse at 0.03-0.14%. 1

Per-Act Transmission Probabilities by Exposure Route

Parenteral Exposures (Highest Risk)

• Blood transfusion with contaminated blood: 95 in 100 (95%) – the most efficient transmission route 1
• Needle sharing among injection drug users: 1 in 150 (0.67%) 1
• Occupational needlestick injury: 1 in 300 (0.3-0.36%) – risk reduced by 81% with prompt post-exposure prophylaxis 1, 2

Sexual Exposures (Variable Risk)

• Receptive anal intercourse: 1 in 10 to 1 in 1,600 (0.5-10%) – the highest-risk sexual act, with substantial variability depending on viral load and presence of STIs 1, 3, 4
• Insertive anal intercourse: 1 in 1,000 to 1 in 10,000 (0.06-0.1%) 5
• Receptive vaginal intercourse (male-to-female): 1 in 200 to 1 in 2,000 (0.05-0.5%) 1, 3
• Insertive vaginal intercourse (female-to-male): 1 in 700 to 1 in 3,000 (0.03-0.14%) 1, 3
• Receptive oral intercourse: approximately 1 in 2,500 (0.04%) – substantially lower risk but not zero 5

Vertical Transmission

• Mother-to-child transmission without antiretroviral therapy: 1 in 4 (25%) 1
• Mother-to-child transmission with antiretroviral therapy and appropriate interventions: less than 2% – represents a 92% risk reduction 1

Critical Risk Modifiers That Dramatically Alter Transmission Probability

Factors That Increase Risk

• High viral load in the HIV-positive partner: Each 10-fold increase in plasma viral load increases heterosexual transmission risk by 2.5-fold 1, 3
• Presence of sexually transmitted infections (STIs) in either partner: Dramatically increases transmission through mucosal inflammation, increased viral shedding, and compromised epithelial barriers 1, 3, 6
• Acute HIV infection in the source partner: Viral loads are extremely high during primary infection, substantially increasing transmission risk 1
• Trauma or bleeding during intercourse: Further elevates transmission probability 3
• Lack of male circumcision: Increases receptive partner risk in heterosexual transmission 3

Factors That Decrease Risk

• Antiretroviral therapy with viral suppression in the HIV-positive partner: Reduces transmission risk by approximately 96% 3, 4
• Pre-exposure prophylaxis (PrEP) in the HIV-negative partner: Provides substantial protection when taken consistently 3, 6
• Consistent condom use: Highly effective barrier method 1
• Combined use of condoms and antiretroviral treatment: Reduces sexual transmission risk by 99.2% 4

High-Risk Populations Requiring Immediate Intervention

Men Who Have Sex With Men (MSM)

• MSM practicing receptive anal intercourse face the highest HIV acquisition risk and require quarterly HIV testing with immediate PrEP consideration 3, 6
• Black MSM are disproportionately affected, accounting for 43% of HIV/AIDS cases among men despite representing only 12% of the US population 6
• Recurrent bacterial STIs in MSM create synergistic increases in HIV transmission risk requiring immediate intervention 3, 6

Heterosexual Exposures

• High-risk heterosexual contact accounts for 80% of HIV cases in women and 16% of cases in men 1
• Injection drug use accounts for 19% of cases in women and 12% in men 1

Prevention Strategies Based on Exposure Type

Post-Exposure Prophylaxis (PEP)

• Initiate PEP within 72 hours (ideally within 1 hour) following high-risk exposure using a 28-day course of combination antiretroviral therapy 3, 2
• Preferred regimens include bictegravir/emtricitabine/tenofovir alafenamide (single tablet once daily) or dolutegravir-based combinations 2
• PEP reduces occupational needlestick transmission risk by 81% when started promptly 2
• Complete the full 28-day course – stopping early eliminates protection 2

Pre-Exposure Prophylaxis (PrEP)

• Offer PrEP immediately to individuals with recurrent bacterial STI diagnosis or ongoing high-risk sexual practices 3, 6
• MSM with bacterial STI diagnosed in the past 12 months should receive PrEP without delay 6

Screening and Monitoring

• Comprehensive STI screening every 3-6 months at all exposure sites (pharynx, rectum, urethra) is essential for high-risk individuals 3, 6
• HIV testing every 3 months minimum for high-risk individuals using tests approved for acute/primary HIV-1 infection detection 6

Common Pitfalls and Critical Caveats

Misconceptions About Antiretroviral Therapy

• HIV can still be detected in genital secretions of patients with undetectable plasma viral loads – all patients receiving therapy can potentially transmit HIV 1
• Treatment interruptions lead to viral load rebound and increased transmission risk 1
• Antiretroviral therapy is not a substitute for risk-reduction behaviors – condoms remain essential 1

Per-Act Risk Variability

• Published per-act risk estimates represent averages and can be highly misleading when applied to specific patients or situations 1
• Substantial heterogeneity exists in per-contact risk – some seroconversions occur after only one or two exposures to receptive anal intercourse 5
• Risk estimates assume constant per-contact infectivity, which may be inaccurate given biological and behavioral variability 1

Prevention Certainty

• The only certain means to prevent sexual transmission are sexual abstinence or sex exclusively with a partner known to be HIV-infected 1
• For injection drug users, the only certain prevention is abstaining from injection drug use or never sharing injection equipment 1