References UK clinical guidance from the NHS, the British HIV Association (BHIVA), and the British Association for Sexual Health and HIV (BASHH). Last updated: April 2026.
If you’ve had a recent exposure and are worried about HIV, it helps to understand how risk is estimated. The actual chance of catching HIV from a single exposure depends on the type of activity, the HIV status and viral load of the other person, whether condoms or PrEP were used, and whether any other infections are present.
Here’s a guide to the numbers behind HIV risk — and more importantly, how to use them to decide what to do next.
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Per-Act Risk Estimates
The figures below come from pooled studies cited in NHS and BHIVA resources. They are averages — real risk depends on many factors.
| Activity | Estimated Risk per 10,000 exposures* |
|---|---|
| Receptive anal sex | Around 138 |
| Insertive anal sex | Around 11 |
| Receptive vaginal sex | Around 8 |
| Insertive vaginal sex | Around 4 |
| Receptive oral sex | Very low — not reliably quantified |
| Sharing injecting equipment | Around 63 |
| Needlestick injury | Around 23 |
*Assumes the source is HIV-positive with a detectable viral load and no condoms or PrEP are used.
Factors That Lower Risk
- Undetectable viral load (U=U): no sexual transmission
- PrEP: more than 99% reduction when taken as prescribed
- Condoms used correctly: around 80-95% reduction
- Circumcision reduces female-to-male vaginal transmission
- No ulcerative STIs present in either partner
Factors That Raise Risk
- Detectable or very high viral load (acute infection or not on ART)
- No condom or PrEP use
- Other STIs, especially syphilis, herpes or gonorrhoea
- Bleeding, trauma or menstruation during sex
- Receptive anal sex
What to Do After a Possible Exposure
- Within 72 hours: If you think you’ve had a risky exposure and are not on PrEP, contact a sexual health clinic or A&E about PEP (post-exposure prophylaxis). Starting PEP within 72 hours can prevent infection.
- Test at 4 weeks: A 4th-generation lab test at 4 weeks is reliable for most people.
- Retest at 3 months if you want confirmation.
- Consider PrEP if you’re likely to be at ongoing risk.
Risk Assessment in Context
Single numbers can be misleading. A 1-in-72 chance (roughly the receptive anal sex figure) sounds different from “138 per 10,000” but they mean almost the same thing. And any risk estimate needs to be adjusted for the specific situation — viral load, protection used, and so on. If you’re worried, the most useful thing you can do is call a sexual health clinic and get an individualised assessment.
Read more: Anal sex and HIV risk | Condoms and HIV | PrEP benefits and risks
Frequently Asked Questions
Can I get HIV from one exposure?
Yes, though it’s statistically uncommon per act without protection. The risk depends heavily on the activity, viral load and protective factors. One exposure is enough if the conditions are right.
Is oral sex a real HIV risk?
Oral sex carries very low HIV risk — so low that reliable estimates cannot be produced from available data. Receiving oral sex is lower risk than giving it. Other STIs transmit more easily through oral sex than HIV does.
How long after exposure should I test?
A 4th-generation lab test at 4 weeks is reliable for most people. Some services recommend a confirmatory test at 3 months.
Related reading
- A to Z of Sex: Sexual Health Terms Explained
- Alcohol and HIV: What You Need to Know
- Anxiety and HIV: Coping and Getting Support
- Are All STIs Curable?
- Blood Problems and HIV: Anaemia, Low Platelets and More
Sources
- NHS — Sexual health conditions
- BASHH — UK clinical guidelines for STI management
- British HIV Association (BHIVA) — HIV treatment guidelines
- Terrence Higgins Trust — sexual health information
- UK Health Security Agency — STI surveillance data
Steve Page is a recognised expert on Sexually Transmitted Diseases (STDs) and STD treatments, having published numerous articles in peer-reviewed journals and presented his research at conferences around the world. He has an in-depth understanding of the latest medical research on STDs, and is an advocate for the development of new treatments and protocols to improve the health of those affected. In addition to his research, he has dedicated his career to understanding the causes and symptoms of STDs, as well as how to best treat those impacted.
