mRNA-1608
Also known as: mRNA-1608-P101
discontinued Moderna (ModernaTX, Inc.)
Last updated:
| Developer | Moderna (ModernaTX, Inc.) |
|---|---|
| Platform | mRNA |
| HSV target | HSV-2 |
| Approach | therapeutic |
| Phase | discontinued |
| Status | Phase 1/2 trial completed April 2025 with positive interim data. Moderna announced in November 2025 that it will not advance the program to Phase 3, ending development. |
| Trials | NCT06033261 |
Moderna's mRNA therapeutic vaccine candidate for people who already have recurrent HSV-2 genital herpes. Its Phase 1/2 trial completed in April 2025 and reported encouraging interim safety, immune-response, and exploratory clinical-endpoint data. In November 2025 Moderna discontinued the program as part of a broader pipeline prioritization; it will not proceed to Phase 3.
What it is
mRNA-1608 was Moderna’s therapeutic herpes vaccine candidate — a vaccine intended not to prevent infection but to help people who are already infected with HSV-2, by reducing the frequency and severity of genital-herpes recurrences. (A therapeutic vaccine treats an existing infection; a prophylactic vaccine tries to prevent one. The two goals are distinct, and a candidate designed for one is not evidence for the other.)
Like Moderna’s other products, it used mRNA delivered in a lipid nanoparticle — genetic instructions that prompt the recipient’s own cells to make selected viral proteins, training the immune system to recognize them. Per the trial’s interim report, mRNA-1608 encoded several HSV-2 antigens, including the glycoproteins gB, gC, and gD and two immediate-early proteins (ICP0 and ICP4).
Where it stands
The candidate was tested in a single Phase 1/2 trial (NCT06033261) in adults aged 18–55 with recurrent HSV-2 genital herpes. The trial began in September 2023 and its status on ClinicalTrials.gov is Completed (primary completion April 25, 2025).
Interim results, presented at IDWeek in October 2025, reported that the vaccine was generally well tolerated, induced antigen-specific antibody and cell-mediated immune responses across dose levels, and — in an exploratory analysis — showed a trend toward delayed and reduced recurrences relative to a control. These are early-stage signals, not proof of efficacy.
In November 2025, Moderna announced it would not advance mRNA-1608 to Phase 3, discontinuing the program as part of a company-wide pipeline prioritization. The available reporting frames this as a strategic/portfolio decision rather than a reported safety or efficacy failure. As of the date above, the program is not active.
Registry snapshot
- Registry status
- Completed
- Phase
- phase 1/2
- Enrollment
- 303 (actual)
- Lead sponsor
- ModernaTX, Inc.
- Started
- Sep 6, 2023
- Primary completion
- Apr 25, 2025
Registry record last updated May 8, 2026.
Background
- How mRNA herpes vaccines work
Several of the most-watched herpes vaccine candidates use mRNA — the same technology behind the COVID-19 vaccines. This explainer covers how an mRNA vaccine delivers instructions for a cell to make selected viral proteins, why several HSV programs chose the approach, and why a platform that works for one disease is not proof it will work for another.
- HSV-1 vs HSV-2: what's the difference?
Herpes simplex virus comes in two types. HSV-1 has traditionally caused oral herpes and HSV-2 genital herpes, but that line has blurred, and either type can infect either site. The distinction still matters for vaccines: a vaccine can protect against one type and not the other, as the Phase 3 Herpevac trial showed. Neither type has an approved vaccine as of 2026.
- Prophylactic vs therapeutic: two very different herpes vaccines
"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.
- What each clinical trial phase does and doesn't prove
A plain-language guide to reading herpes (HSV) vaccine news by trial phase. Each phase answers a different question, and early-phase results say nothing about whether a vaccine prevents disease. A trial existing or finishing is not the same as a vaccine working — the Phase 3 Herpevac trial ran to completion but missed its main goal, and as of 2026 no HSV vaccine has cleared Phase 3 to approval.
Key literature
Sources
- A Study of mRNA-1608, a Herpes Simplex Virus-2 (HSV-2) Therapeutic Candidate Vaccine, in Healthy Adults With Recurrent HSV-2 Genital Herpes (NCT06033261)
- mRNA-1608, an mRNA-Based Therapeutic Genital Herpes Vaccine Candidate: Interim Safety, Immunogenicity and Clinical Endpoint Results from a Phase 1/2 Trial (abstract 642)
- Moderna Cans 3 mRNA Vaccines, Secures $1.5B Loan in Quest for 10% Growth