HIV Risk Calculator
Calculate HIV transmission risk per exposure type, assess modifying factors, and determine PEP (Post-Exposure Prophylaxis) indication. This calculator provides evidence-based risk estimates and helps guide decision-making for HIV prevention.
Exposure Assessment
Modifying Factors
Understanding HIV Transmission
Factors Affecting Transmission
Increase Risk
- • High viral load in source
- • Acute HIV infection (very high viral load)
- • Presence of STIs (ulcerative > non-ulcerative)
- • Traumatic sex/tissue damage
- • Uncircumcised male (insertive)
- • Anal sex (higher than vaginal)
Decrease Risk
- • Undetectable viral load (U=U)
- • Consistent condom use
- • Male circumcision
- • PrEP use by HIV-negative partner
- • ART by HIV-positive partner
- • No concurrent STIs
PEP Protocol
Post-Exposure Prophylaxis Timeline:
- 0-2h:Optimal window - highest effectiveness
- 2-24h:Still highly effective - seek care urgently
- 24-72h:Can still be effective - worth starting
- >72h:Generally not recommended - effectiveness uncertain
HIV Testing After Exposure
Even with PEP, follow-up HIV testing is essential:
- •Baseline: HIV test before starting PEP
- •6 weeks: Early detection possible with 4th generation tests
- •3 months: Standard window period endpoint
- •6 months: Final confirmatory test (conservative approach)
Frequently Asked Questions
What does "U=U" mean?
U=U stands for "Undetectable = Untransmittable." People with HIV who take their antiretroviral medications as prescribed and maintain an undetectable viral load have effectively zero risk of sexually transmitting HIV to their partners. This is supported by major studies including PARTNER and Opposites Attract.
How effective is PEP at preventing HIV?
When started within 72 hours of exposure and taken correctly for 28 days, PEP is highly effective, reducing HIV transmission risk by more than 80-90%. Effectiveness is highest when started as soon as possible after exposure, ideally within 2 hours. Animal studies suggest near 100% effectiveness when started immediately.
Can I get HIV from oral sex?
The risk of HIV transmission through oral sex is extremely low but not zero. Receptive oral sex (performing oral sex on someone with HIV) carries a risk of approximately 0.01% per exposure. Insertive oral sex (receiving oral sex) has an even lower risk, estimated at 0.005% per exposure. Risk increases with mouth ulcers, bleeding gums, or genital sores.
Why is receptive anal sex the highest risk?
Receptive anal intercourse carries the highest risk (~1.38% per exposure) because the rectal lining is thin and more susceptible to micro-tears, providing easier access for the virus to enter the bloodstream. The rectum also has a high concentration of immune cells that HIV can infect. This risk is significantly higher than receptive vaginal intercourse (0.08%) or insertive sex.
Does circumcision really reduce HIV risk?
Yes, multiple randomized controlled trials in Africa showed that male circumcision reduces the risk of female-to-male HIV transmission by approximately 50-60%. This protection is for the circumcised male during insertive vaginal or anal sex. Circumcision does not protect female partners of HIV-positive men, nor does it protect the circumcised male during receptive anal sex.
What is the difference between PEP and PrEP?
PEP (Post-Exposure Prophylaxis) is emergency medication taken after a potential HIV exposure within 72 hours, continued for 28 days. PrEP (Pre-Exposure Prophylaxis) is daily medication taken before exposure to prevent HIV infection in people at ongoing risk. PrEP is >99% effective when taken daily, while PEP is 80-90% effective when started promptly after a single exposure.
How do STIs increase HIV transmission risk?
STIs increase HIV transmission risk by approximately 2-5 times. Ulcerative STIs (herpes, syphilis, chancroid) create breaks in the skin that allow easier viral entry. Inflammatory STIs (gonorrhea, chlamydia) increase the concentration of HIV-susceptible immune cells at the infection site. In HIV-positive people, STIs can increase viral load in genital secretions.
Can I get HIV from a needlestick injury?
The risk of HIV transmission from a needlestick injury is approximately 0.23% (about 1 in 435). Risk is higher if the needle was used for a blood draw, the source patient has a high viral load, the needle penetrated deeply, or visible blood was on the device. Healthcare workers should seek immediate PEP evaluation after any occupational exposure. Most exposures do not result in transmission.