Short answer
The clearest number comes from one trial: over 8 months, HSV-2 passed to 3.6% of susceptible partners on placebo and 1.9% when the infected partner took daily valacyclovir. That is a group result in specific couples, not a personal annual risk. No study can give you a percentage for your own relationship, so treat the numbers below as context, not a forecast.
Checked September 8, 2026 · Sources: CDC, NEJM, JAMA, listed at the end · This site is run by an employee of the company that owns PositiveSingles. Why that matters
Start with what was measured
Corey and colleagues enrolled 1,484 couples in which one partner had symptomatic genital HSV-2 and the other was susceptible, and measured new HSV-2 infections over 8 months. Both groups received safer-sex counseling and were offered condoms.
Fewer partners acquired HSV-2 with daily valacyclovir
One eight-month trial · overall HSV-2 acquisition · each rung = 0.1 percentage point
Corey et al. · New England Journal of Medicine · 2004
Read the data & limits
| Source partner's treatment | New infections | Reported rate |
|---|---|---|
| Placebo | 27 / 741 | 3.6% |
| Valacyclovir | 14 / 743 | 1.9% |
The trial enrolled 1,484 immunocompetent, heterosexual, monogamous couples. One partner had symptomatic genital HSV-2; the other was susceptible. The infected partner received valacyclovir 500 mg daily or placebo. Both groups received safer-sex counseling and were offered condoms.
The outcome includes infections with or without symptoms. The hazard ratio was 0.52 (95% confidence interval 0.27 to 0.99). The reported percentages are rounded; their difference is 1.7 percentage points.
This is not a comparison with unprotected sex, a sex-specific estimate, or a prediction for your relationship. It should not be converted into a universal annual rate. Medication decisions belong with your clinician.
The difference in the rounded percentages is 1.7 points. The trial's hazard ratio was 0.52, which describes the relative hazard over follow-up. Those are two descriptions of the same study, not interchangeable personal forecasts.
What medication can change
The CDC treatment guidance includes daily valacyclovir as an option to reduce transmission in couples like those studied, and antivirals can also help manage recurrences. Medication does not remove HSV from the body or guarantee that transmission will not happen. Treatment decisions differ for genital HSV-1 and for HSV-2 diagnosed by a blood test in someone without symptoms, so ask your clinician which evidence applies to you rather than assuming everyone needs the same plan.
What condoms do
A pooled analysis of 6 prospective studies found that people who used condoms for every sex act had a 30% lower risk of HSV-2 acquisition than people who never used them; the analysis included 5,384 people, hazard ratio 0.7 (95% CI 0.4 to 0.94). Condoms do not cover every area that can shed virus, which is why they reduce risk without removing it. The condom guide covers what they reach.
Why we don't multiply the percentages
The medication trial and the condom analysis used different designs, and multiplying their relative reductions would produce a new estimate that neither study directly measured. The placebo group was not a "no precautions" group either. A workable plan can still combine condoms, avoiding contact during symptoms, and medication when appropriate, and a clinician can help you discuss the remaining risk without pretending it is known to a decimal place.
Genital HSV-1 needs its own evidence
The CDC notes that genital HSV-1 generally causes fewer recurrences and less shedding than genital HSV-2, and that shedding decreases during the first year after genital HSV-1 infection. A 2022 study detected genital HSV-1 shedding on 12.1% of sampled days at month 2 and 7.1% at month 11. That is useful for counseling, but it does not supply an annual transmission rate, and the CDC says suppressive treatment to prevent genital HSV-1 transmission has not been studied. We do not relabel it "1% per year" or reuse the HSV-2 trial result. Asymptomatic shedding is one reason transmission happens between outbreaks; the shedding guide has the repeated-swab data.
How to talk about these numbers
When you disclose, keep it simple, and use this only if it describes your actual treatment.
YouI have genital HSV-2. I take daily medication, and my clinician has helped me understand how to reduce the chance of transmission. I wanted you to know before we have sex, and I'm happy to answer questions or give you time to think.
A partner may want details, time, or a conversation with a clinician. You can share the study without calling its result your personal risk. And avoid sexual contact from the moment you feel prodrome symptoms, tingling, itching, or discomfort, until the skin looks normal again; the CDC includes that in its prevention advice. If a partner develops symptoms, encourage them to get medical advice rather than diagnosing from a photo.
Sources
- Corey L et al., New England Journal of Medicine, 2004. Once-daily valacyclovir in 1,484 discordant couples over 8 months: 27 of 741 transmissions on placebo, 14 of 743 with treatment. DOI 10.1056/NEJMoa035144.
- Martin ET et al., Archives of Internal Medicine, 2009. Pooled analysis of 6 studies, 5,384 people; consistent condom use hazard ratio 0.7 (95% CI 0.4–0.94).
- Tronstein E et al., JAMA, 2011. Genital shedding among symptomatic and asymptomatic persons with HSV-2. DOI 10.1001/jama.2011.420.
- CDC, STI Treatment Guidelines, genital herpes. Suppressive therapy, condoms, and prevention advice. Consulted September 8, 2026.