Prophylactic vs therapeutic: two very different herpes vaccines
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"A herpes vaccine" can mean two different things. A prophylactic (preventive) vaccine is given to people who are not infected, to stop them getting herpes simplex virus (HSV) at all. A therapeutic vaccine is given to people who are already infected, to reduce outbreaks and viral shedding. The two use different trials, endpoints, and populations — so the first question to ask of any candidate is which goal it targets. Neither type is approved.
The short answer
When people say “a herpes vaccine,” they can mean one of two things, and the two have different goals. Herpes simplex virus (HSV) is the virus that causes cold sores and genital herpes; it comes in two closely related types, HSV-1 and HSV-2. A prophylactic vaccine — “prophylactic” simply means preventive — is given to people who do not have HSV, with the goal of stopping them from becoming infected in the first place. A therapeutic vaccine is given to people who already have HSV, with the goal of reducing how often the infection flares up and how much virus they shed. [S1][S3]
Because the two aim at different problems, they are tested in different ways: they enroll different people, and they measure success against different endpoints (the pre-defined outcome a trial is designed to detect). That means the kind of vaccine a candidate is — preventive or therapeutic — changes how any claim about it should be read. [S1][S3]
Prophylactic: preventing infection
A prophylactic vaccine is given to people who are HSV-negative, before they are ever exposed. Its job is to keep the virus from establishing an infection at all — including latency, the lifelong dormant state HSV settles into inside nerve cells after the first infection, from which it can later reawaken. A prophylactic vaccine is judged a success if vaccinated people go on to have fewer new infections, or fewer cases of disease, than unvaccinated people. [S1][S3]
One candidate in this category is BNT163, a preventive vaccine from BioNTech that, as of the cited 2024 reviews, was in a Phase 1 trial — the earliest, small, first-in-human safety stage of clinical testing. [S3]
The clearest illustration of how a preventive trial reads comes from an earlier program: the Herpevac trial of GlaxoSmithKline’s glycoprotein-D (gD2) vaccine. This was a large Phase 3 field trial — Phase 3 being the large, late-stage efficacy stage — in 8,323 women who were negative for both HSV-1 and HSV-2 antibodies. Its result shows why the HSV-1/HSV-2 distinction matters: the vaccine protected against HSV-1 but not HSV-2. Efficacy against HSV-1 genital disease was 58% and against HSV-1 infection 35%, while no protective effect against HSV-2 infection or disease was observed. [S5]
Therapeutic: controlling an existing infection
A therapeutic vaccine is aimed at the population already infected with HSV. The goal is not prevention but control: to alleviate symptoms, slow disease progression, and suppress recurrences — the repeat outbreaks that happen when latent virus reactivates (reawakens from dormancy) and travels back to the skin or mucous membranes. [S3] A specific target is viral shedding, the release of infectious virus (often without visible symptoms); shedding is what allows the virus to be passed to other people, so reducing it matters both for the patient and for onward transmission. [S3]
The best-documented therapeutic example is GEN-003, a subunit vaccine originally from Genocea. In Phase 1 and Phase 2 trials it significantly reduced genital lesions and viral shedding, yet Genocea stopped funding the program in 2017 to focus on cancer vaccines; the candidate has not advanced further. [S1]
Another therapeutic candidate is mRNA-1608, an mRNA vaccine from Moderna developed with the University of Pennsylvania — see its candidate page for its current, sourced status.
Why the distinction matters when reading the news
The practical takeaway is that a candidate which helps with one goal will not automatically help with the other. The underlying biology, the trial design, and the target population all differ between preventive and therapeutic approaches. So when a headline mentions “a herpes vaccine,” the first question worth asking is: prophylactic or therapeutic? [S1][S3]
Some programs are explored for both purposes — in animal studies, for instance, the trivalent mRNA approach behind BNT163 showed both preventive and therapeutic effects — and public-health bodies have described a need for both prophylactic and therapeutic HSV vaccines. [S1][S3]
What this does and does not mean
Neither a prophylactic nor a therapeutic HSV vaccine is approved. As of the most recent cited review (2024), an effective preventive or therapeutic HSV vaccine remained unavailable, and that remains the case as of this page’s last update. [S1] Progress on one type does not imply progress on the other. Keeping the two straight — together with the distinction between HSV-1 and HSV-2, and between a result seen in animals (preclinical) versus one seen in people (in-human) — is essential to reading any claim about HSV vaccines accurately. [S1]