HSV-1 Vaccine: Current Status and Clinical Trials

No approved vaccine exists for herpes simplex virus type 1, despite the virus infecting roughly two-thirds of the global population under age 50. Several vaccine candidates targeting HSV-1 or its close relative HSV-2 are in preclinical or early clinical development, spanning live-attenuated viruses, mRNA platforms, subunit formulations, and even gene-editing therapies aimed at eliminating latent virus from nerve tissue. The science has advanced considerably in the past decade, but the path from promising animal data to a licensed human vaccine remains long and littered with high-profile failures.

The Scale of HSV-1 Infection

HSV-1 is one of the most widespread human pathogens on the planet. Global estimates put the number of people carrying the virus at about 3.7 billion among those aged 0 to 49, a prevalence of roughly 67%.¹ Most infections are oral, but genital HSV-1 has been increasing steadily in many countries. Among people aged 15 to 49, an estimated 376 million had genital HSV-1 infection in 2020, giving a genital prevalence of about 10%.² Together, oral and genital infections caused by HSV-1 and HSV-2 affect several billion people worldwide, creating enormous demand for a preventive or therapeutic vaccine.

What Makes HSV-1 Such a Difficult Vaccine Target

After the initial infection, HSV-1 retreats into sensory nerve cells, primarily in the trigeminal ganglia for oral infections and the dorsal root ganglia for genital infections, where it persists for life. During this latent phase, the virus produces very little protein, making it nearly invisible to the immune system. The key viral product during latency is the latency-associated transcript (LAT), along with certain small RNAs that help regulate whether the virus stays dormant or reactivates.³

The LAT does more than simply keep the virus quiet. Research in mice has shown that during latency, most HSV-1-specific immune cells in the ganglia become functionally exhausted, producing less of the signaling molecules needed to kill infected cells. The LAT appears to drive this exhaustion by upregulating immune-checkpoint molecules that tell the immune system to stand down.⁴ This means that even a robust vaccine-induced immune response has to contend with a virus that actively disarms the immune cells sent to patrol its hiding place.

Tissue-resident memory T cells that persist at the site of past outbreaks do help control viral shedding between episodes, likely through secretion of interferon-gamma.⁵ These cells are thought to be critical for preventing reactivation, and some newer vaccine strategies aim specifically to boost their numbers. In animal studies, blocking immune-checkpoint pathways like PD-1 and LAG-3 alongside vaccination restored the function of these exhausted T cells and reduced ocular herpes disease.⁶ Whether that approach can be safely replicated in people remains to be seen.

Lessons from the Herpevac Trial

The most instructive failure in herpes vaccine history is probably the Herpevac Trial for Women, a large study of a subunit vaccine containing HSV-2 glycoprotein D (gD) with an adjuvant. Two earlier trials in couples where one partner had herpes had suggested the vaccine was about 73 to 74% effective against genital disease in women who had never been exposed to either HSV-1 or HSV-2. But when a larger trial enrolled only women seronegative for both virus types, the overall result was disappointing: vaccine efficacy against genital herpes disease was just 20%, and the confidence interval crossed zero.⁷

A surprising twist emerged from that trial. While the vaccine failed against HSV-2, it showed about 58% efficacy against genital disease caused by HSV-1 and about 35% protection against HSV-1 infection overall.⁷ This was unexpected for a vaccine built from an HSV-2 protein, and it shifted thinking in the field. It suggested that partial cross-type protection is possible, that antibodies alone might not be enough, and that relying on a single glycoprotein might be too narrow a strategy. Every major candidate now in development reflects at least one of those lessons.

Live-Attenuated Candidates and Cross-Type Protection

One of the more active areas of research involves live-attenuated HSV-1 strains engineered to replicate poorly and avoid infecting nerve tissue. The VC2 vaccine, an HSV-1 strain with specific mutations that prevent it from entering neurons, has shown promising results in guinea pigs challenged with HSV-2. Vaccinated animals had less severe acute and recurrent disease, reduced viral shedding, and lower levels of virus in their nerve ganglia compared to unvaccinated controls.⁸ Later work showed that a modified version of VC2 given by intradermal injection induced neutralizing antibodies against HSV-2 in all vaccinated animals.⁹

Another non-neuroinvasive HSV-1 strain called R2 has been tested similarly. In guinea pigs, intradermal R2 vaccination produced neutralizing antibody titers about three-fold higher than a gD2 subunit vaccine with adjuvant, and the intradermal route again outperformed other delivery methods.¹⁰ The cross-protection these HSV-1 vaccines offer against HSV-2 is particularly encouraging because it opens the possibility of a single vaccine addressing both virus types, though the protection against the mismatched type is consistently weaker.

Live-attenuated vaccines have an inherent advantage in ocular herpes models. When mice were challenged with HSV-1 to simulate eye infection, a live-attenuated vaccine completely preserved visual acuity and prevented corneal scarring, whereas a subunit vaccine did not prevent vision loss.¹¹ This likely reflects the broader immune response a live virus can trigger compared to a single-protein subunit approach.

Antibody Strategies Beyond Simple Neutralization

Traditional vaccine design assumes that neutralizing antibodies, which directly block the virus from entering cells, are the key to protection. But one of the more provocative findings in herpes research is that a different type of antibody response may matter more. A vaccine based on a single-cycle HSV-2 virus with its gD gene deleted (called ΔgD-2) produced low neutralizing antibody levels but high levels of antibodies that flag infected cells for destruction by immune cells, a mechanism known as antibody-dependent cellular cytotoxicity. In mice already carrying HSV-1, this vaccine significantly boosted total HSV-specific antibody titers and provided complete protection from death after HSV-2 challenge.¹²

New structural biology work is adding another dimension. Researchers have recently solved high-resolution structures of HSV-2 glycoprotein B (gB), one of the main proteins the virus uses to fuse with host cells. Both protein-based and mRNA-based vaccines using a stabilized prefusion form of gB elicited strong immune responses in mice, and the prefusion version triggered antibodies with greater ability to recruit immune-cell killing compared to the postfusion form, even though neutralizing titers were similar. This suggests that the shape of the vaccine antigen matters for the quality, not just the quantity, of the immune response.

mRNA Vaccines Targeting Multiple Glycoproteins

The success of mRNA vaccines against COVID-19 has accelerated interest in applying the same technology to herpes. One approach uses nucleoside-modified mRNA encoding three HSV-2 glycoproteins: gC, gD, and gE. In mice, this trivalent mRNA vaccine protected against both HSV-2 and HSV-1 genital lesions and latent infection. However, the cross-type protection was notably weaker against HSV-1. Analysis showed that antibody titers against gC and gE were significantly higher for the matched HSV-2 proteins than for their HSV-1 counterparts, while gD responses were similar across types, likely reflecting the higher amino acid similarity between gD1 and gD2 (about 82%) compared to gC (65%) and gE (73%).¹⁰

Why use three glycoproteins instead of one? Each plays a distinct role in how the virus interacts with the immune system. gC helps the virus evade complement, a part of innate immunity; gD is the main receptor-binding protein; and gE helps the virus dodge antibodies by binding a component of immunoglobulin. A vaccine targeting all three simultaneously aims to close off multiple escape routes at once. Whether this strategy will translate from mice to humans is the central question, and no mRNA herpes vaccine has yet entered a large efficacy trial.

Therapeutic Vaccines for People Already Infected

Because billions of people are already living with HSV, a preventive vaccine alone would take decades to change the global burden. Therapeutic vaccines aim to reduce outbreaks and viral shedding in people who are already infected. The furthest along was GEN-003, a protein subunit candidate tested against genital HSV-2. In a randomized trial, the best-performing dose reduced viral shedding by about 42% immediately after dosing and also lowered lesion rates, recurrence rates, and duration of outbreaks.¹³ These reductions in shedding and lesions persisted for up to 12 months after immunization.¹⁴

GEN-003’s development was eventually halted for business reasons rather than safety concerns, but its results validated an important clinical endpoint. Shedding rate, measured by daily self-swabs, has been shown to strongly correlate with lesion rates and recurrence frequency, making it a useful surrogate endpoint for phase 2 trials.¹⁵ This endpoint is now widely accepted in the field and lowers the bar for testing future therapeutic candidates, since measuring shedding requires smaller trials and shorter follow-up than waiting to count clinical outbreaks.

Gene Editing to Destroy Latent Virus

The most ambitious approach does not try to boost the immune system at all. Instead, it aims to find the latent virus hiding in nerve ganglia and physically destroy its DNA. Fred Hutchinson Cancer Center researchers have been developing engineered meganucleases, delivered to neurons via harmless viral vectors (AAVs), that cut the HSV genome in latently infected cells. In mice, AAV-delivered meganucleases eliminated over 90% of latent virus from the superior cervical ganglia.¹⁶

Subsequent experiments tested combinations of AAV serotypes to reach more neurons. In a vaginal infection model, a triple combination of AAV vectors achieved a remarkable 97.7% reduction in HSV viral load in the dorsal root ganglia. In an orofacial infection model treated in parallel, reductions reached 89% in superior cervical ganglia and 61% in trigeminal ganglia. Separately, a CRISPR-based approach in rabbits with latent HSV-1 eye infections achieved a 50% reduction in viral load and an 80% reduction in LAT RNA, the main transcript produced during latency.¹⁷

Gene editing is still years from human trials and faces significant challenges around delivery efficiency, off-target effects, and manufacturing scale. But the data represent a genuinely different kind of cure: not controlling the virus, but erasing it. If even partial elimination of latent virus translates into reduced reactivation and shedding in humans, gene editing could complement conventional vaccines rather than compete with them.

Protecting Newborns Through Maternal Immunization

Neonatal herpes is rare but devastating, with high rates of brain damage and death when a baby is infected during delivery. One strategy bypasses the challenge of vaccinating newborns directly by immunizing mothers during pregnancy. In animal studies, vaccination of pregnant mice with a replication-defective HSV-2 candidate called dl5-29 led to transfer of HSV-specific antibodies into the pups, protecting them from neurological disease and death after herpes challenge.¹⁸

The transfer of protective maternal antibodies across the placenta appears to be robust. Even under conditions of acute psychological stress in animal models, the passive transfer of HSV-specific antibodies from mother to offspring persisted and was sufficient to prevent the virus from spreading systemically in the newborn.¹⁹ Maternal immunization is a well-established concept for other infections and could become one of the first practical applications of a herpes vaccine if candidates advance to clinical trials in pregnant women.

How Much Difference Could a Vaccine Make at the Population Level

Mathematical modeling provides a window into what even a moderately effective vaccine could accomplish. A 2024 analysis projected the impact of a hypothetical HSV-1 vaccine introduced in the United States in 2030. If infants were vaccinated with a product offering 70% efficacy and lifelong protection, the model estimated that by 2075, oral HSV-1 incidence would drop by about 47% and genital HSV-1 incidence by about 58%, with roughly 28.5 million cumulative infections prevented over that period.²⁰ Those are substantial numbers, and they assume a vaccine that is good but far from perfect. The genital incidence drops faster than oral incidence because genital HSV-1 is primarily acquired in adulthood, giving a childhood vaccine more time to work before the period of peak exposure.

These models also highlight a practical reality: even a highly effective prophylactic vaccine would take decades to substantially reduce the prevalence of a virus that already infects most of the world’s population. Therapeutic vaccines or gene-editing cures for people who already carry the virus would be needed alongside prevention to meaningfully change the global picture within a generation.

The Animal Model Problem

One persistent headache in herpes vaccine development is the gap between animal results and human outcomes. Mice are the workhorse of early testing, but they do not spontaneously shed virus or develop recurrent symptomatic disease the way humans do. Guinea pigs and rabbits are better in this regard, as they experience spontaneous reactivation and recurrent disease that more closely resembles the human pattern. However, none of these animal models can mount T cell responses against the specific human immune markers that a therapeutic vaccine would need to engage.²¹

This limitation has real consequences. Vaccines that looked excellent in mice, including several based on single glycoprotein approaches, have failed or underperformed in human trials. The Herpevac results are the most prominent example, but they are not the only one. Rabbits transgenic for certain human immune genes have partially closed this gap, and some promising checkpoint-blockade-plus-vaccine strategies have been tested in these animals.⁶ Still, every candidate discussed in this article carries an asterisk: the animal data are encouraging, but prediction of human efficacy remains unreliable until large-scale human trials are completed.

The Possible Connection to Alzheimer’s Disease

One area of research that could eventually broaden the case for an HSV-1 vaccine involves the brain. HSV-1 DNA has been found in the brains of about two-thirds of people with Alzheimer’s disease and a similar fraction of aged individuals without dementia. In carriers of a specific genetic variant (the APOE4 allele), the presence of HSV-1 in the brain appears to be a strong risk factor for developing Alzheimer’s. An older mouse study found that vaccination with an HSV-1 glycoprotein formulation dramatically reduced the virus’s ability to establish latent infection in the brain: HSV-1 was detected in 41% of unvaccinated mice but only 7% of vaccinated animals.²²

This does not prove that an HSV-1 vaccine would prevent Alzheimer’s, and the hypothesis remains contested. But it illustrates how the stakes of an effective HSV-1 vaccine could extend well beyond cold sores and genital herpes. If future epidemiological and clinical data strengthen the link, the public health calculus for prioritizing HSV-1 vaccine development would shift substantially. Adjuvant research is also progressing in ways that could improve any future candidate. Intranasal delivery of gD2 combined with novel carrier particles, for instance, induced strong mucosal antibodies and a more balanced immune response compared to standard formulations in animal models, pointing toward delivery methods that better match the mucosal surfaces where herpes typically enters the body.²³