Background

HSV-1 infects ~67% of the global population under 50; HSV-2 affects ~11%. Both establish lifelong latency with no cure. Despite decades of attempts, no approved HSV vaccine exists. The most advanced candidate — GSK's Herpevac (gD/AS04) — failed in a Phase 3 trial of 8,000+ women. Understanding why is the key to what comes next.

Why Herpevac Failed

The failure was not a lack of antibodies — it was the wrong kind. Herpevac induced neutralising IgG1 antibodies exclusively, but women with natural chronic HSV-2 infection were protected by a fundamentally different immune mechanism: antibody-dependent cell-mediated cytotoxicity (ADCC). Vaccinated women showed significantly lower ADCC compared to naturally infected women (P < .001). Single-antigen gD vaccines cannot replicate the polyfunctional antibody breadth of natural infection.

Compounding this: HSV actively evades immune killing via gE and gI glycoproteins, which function as viral Fc receptors that bind and inactivate host antibodies, blocking ADCC. Including gD in vaccines may paradoxically facilitate immune escape. Additionally, subunit vaccines fail to generate CD8+ tissue-resident memory T-cells (TRM) in the genital mucosa — the exact anatomical site where HSV establishes infection.

Hypothesis

A multivalent mRNA vaccine encoding gC, gD, gB, and gE-deleted constructs — deliberately excluding intact gE/gI — administered intramuscularly with a mucosal boost in HSV-1 seropositive adults will elicit superior ADCC responses and genital tract CD8+ TRM cells compared to single-antigen gD vaccination, reducing HSV-2 acquisition by 60% over 18 months.

Supporting Evidence

Trivalent and quadrivalent mRNA vaccines protect 98-100% of mice and guinea pigs against HSV-2 genital disease, reduce vaginal viral shedding, and prevent dorsal root ganglia latency in 97-100% of animals. These outperform protein subunit approaches on neutralising titres, B cell memory (1+ year), and T cell responses. Delta-gD-2 vaccines that exclude gD elicit non-neutralising FcR-activating antibodies mediating robust ADCC and protection against lethal HSV challenge in animal models.

The Field

Moderna and BioNTech are now leading HSV vaccine development following GSK's Phase 2 failure of their protein-based candidate. The pivot to mRNA platforms and multivalent targeting is well underway — this hypothesis represents the mechanistically optimal next iteration.

Grounded via AUBRAI. Key references: PMC10205893, pubmed/35834278, pubmed/32817296, journal.ppat.1008795, JCI152310