The need for a herpes vaccine is clear: about half a billion people worldwide between the ages of 15-49 have genital herpes infection caused by either HSV-1 or HSV-2, according to the World Health Organization (WHO). In the United States alone, an estimated 1 in 6 adults have genital herpes, with around 300,000 new infections diagnosed each year.

While HSV is typically a mild infection, there are potential health risks associated with it, including neonatal herpes, a serious and sometimes fatal condition that occurs when HSV is passed to an infant during delivery. Another concern is the increased risk of HIV infection. The risk of getting HIV (if exposed) is increased 2-3 fold for someone with genital HSV-2 infection.

Researchers have been working on developing herpes vaccines for decades. There have been a number of clinical trials aimed at testing both therapeutic (intended to reduce recurrences and viral shedding in people who are already infected with HSV) and preventive (designed to prevent infection) vaccine candidates. The largest to date was the Herpevac Trial for Women, a clinical trial done with over 8,300 women who were uninfected with either HSV-1 or HSV-2. The results of this prophylactic vaccine were mixed: the vaccine was effective only in preventing HSV-1, not HSV-2. Because of these results, development of the vaccine was abandoned after Phase III of the clinical trial.

How do clinical trials work?

Before a treatment regimen or vaccine can become standard, it must go through a clinical trial. Clinical trials test if a potential treatment or vaccine is safe and effective in humans. Clinical trials go through a series of phases, starting with a smaller group of patients and expanding to a much larger group. The video explains the three main phases of a clinical trial.

Herpes Vaccine Clinical Trials

There are currently both preventive and therapeutic vaccines under development. While the primary focus is on HSV-2, the primary cause of genital infection, HSV-2 vaccines may also have benefits in preventing or treating HSV-1 infection. In addition to work being done in the preclinical stage, there are several vaccines in clinical trials. More information on those listed below and other clinical trials can be found at, a resource of the National Library of Medicine.

Phase I

  • HSV529: Clinical trials for this HSV-2 vaccine candidate are studying both preventive and therapeutic applications. HSV529 uses a replication-defective mutant virus, which is genetically altered to prevent the virus from replicating. In the first phase, researchers are studying the safety of HSV529 vaccine and the ability of the vaccine to elicit immune responses to HSV-2 including virus-specific antibodies and T cell responses to the virus.

Phase II

  • GEN-003: Phase II trials are ongoing for GEN-003, a protein subunit vaccine that uses two antigens (a foreign substance that induces an immune response in the body) and an adjuvant (a substance that enhances the body’s immune response to an antigen) to produce an immune response. Early results show that the vaccine is effective in reducing the recurrence of lesions and reducing viral shedding—the first demonstration of reduced viral shedding by a therapeutic vaccine.
  • VCL-HB01: This therapeutic DNA vaccine for individuals with HSV-2 is also in ongoing phase II trials. A phase 2 trial HSV-2 infected adults has been underway since September 2016 to evaluate the recurrence rates over a 12-month period following vaccination.

For more on the current status of herpes vaccine development, including vaccine candidates in the preclinical phase, see this article prepared for the WHO Product Development Vaccine Advisory Committee.