G103

What it is

G103 is an investigational vaccine made of three recombinant HSV-2 proteins: truncated UL19 and gD2, and full-length UL25. In its preclinical description, the proteins were combined with an adjuvant, glucopyranosyl lipid A (GLA), formulated in a stable emulsion. A 2025 trial report describes G103, like HSV529, as an investigational therapeutic vaccine for genital herpes. A 2023 nonclinical report found G103 with GLA-SE, and HSV529, safe and well tolerated in repeated-dose toxicity studies in guinea pigs and rabbits.

Where it stands

A ClinicalTrials.gov search for G103 on 2026-09-28 returned one record, NCT04222985, a Phase 1 trial listed as terminated, and no record with an ongoing status. NCT04222985, sponsored by Sanofi Pasteur, is registered as a Phase 1 study of investigational HSV-2 vaccines in adults with recurrent genital herpes; its intervention list gives numbered HSV-2 formulations and a sodium chloride comparator and does not name G103 in the record text provided here. The trial started in 2020, lists 24 participants as actual enrollment, and ended in 2021 for business reasons, with Phase 2/Part B not conducted after the last patient enrolled in Phase 1/Part A; no results are posted on the registry.

A 2025 publication reports a randomized, placebo-controlled trial of combinations of G103 and HSV529 with the GLA-SE adjuvant on a two-dose schedule in 24 participants; the abstract gives no registry number, so its match to NCT04222985 is not confirmed by these sources. In that trial, no immediate unsolicited adverse reactions were observed, most injection site reactions were grade 2 with one grade 3 swelling, and the authors judged the safety profiles adequate. All participants had immune responses to HSV-2 at baseline; among G103 recipients, CD4 T cells specific to gD2, UL19 and UL25 rose after the first dose, most strongly for gD2, and binding and neutralizing antibodies rose most for UL25, without the abstract saying which vaccine groups the antibody results refer to.

In a 2016 mouse study, G103 vaccination produced binding and neutralizing antibodies and T cell responses, expanded T cells induced by previous HSV-2 infection, and as a prophylactic completely protected mice against lethal intravaginal HSV-2 infection. In the guinea pig model of recurrent HSV-2 infection, therapeutic immunization with G103 was about 50% effective in reducing lesions per animal and the overall lesion score.

What this does and doesn’t mean

As of 2026-09-28, the registry search returned one record (NCT04222985), a Phase 1 study that stopped before its Phase 2 part, and no record with an ongoing status. The published human results describe safety and immune responses, not protection: rising T cells or antibodies do not show that a vaccine reduces herpes outbreaks or shedding. Most of the human safety findings are reported for the vaccine candidates together rather than for G103 alone. The mouse protection and guinea pig lesion-reduction results are animal findings and do not show that G103 works in people.