mRNA-1608

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.