Clinical trials of prophylactic and therapeutic herpes simplex virus vaccines

Herpes simplex virus type 2 (HSV-2) is a suitable target for a vaccine, despite available antiviral therapies, because the virus causes lifelong infection and significant medical and psychosocial morbidity. A vaccine has the potential to reduce HSV acquisition, disease severity and the number of cases of neonatal herpes. It could also reduce transmission of HIV, which is epidemiologically linked to HSV. Prophylactic vaccines for HSV-2 must give broad and durable immunity across all mucosal surfaces to be effective. This is a significant challenge, as the major determinants of effective immunity have not yet been identified. Even if full protection cannot be achieved, vaccines would still be useful if they could increase the threshold of infection, or prevent clinical disease. However, it is possible that a vaccine could reduce symptomatic disease, but not eliminate asymptomatic shedding, which could inadvertently increase transmission from individuals who believe they are not infectious. Investigated prophylactic vaccines for HSV-2, including subunit vaccines encoding HSV glycoproteins packaged with adjuvants, have shown some benefits. The Chiron gD2gB2-MF59 vaccine gave transient protection of less than 6 months. The GlaxoSmithKline gD2-alum MPL vaccine conferred a 73-74% reduction in acquisition of symptomatic HSV-2 disease and a 38-42% reduction in the acquisition of HSV-2 infection in HSV-seronegative women, but gave no protection in men or HSV-1 seropositive women. Therapeutic vaccines aim to prevent HSV recurrences or minimise disease severity and duration, thereby reducing transmission. Research indicates that to be effective, therapeutic vaccines need to stimulate strong cell-mediated immune responses. Vaccines have induced HSV-specific antibody responses alone but have failed to protect recipients from recurrences. Further research is needed to define determinants of immunity to HSV-2, including identifying HSV-2 antigens, in order to design more effective vaccines.