An investigational vaccine intended to prevent genital herpes disease in women was found to be moderately effective against one of the two types of herpes simplex viruses that cause genital herpes, according to an analysis published January 5 in the New England Journal of Medicine (NEJM). Overall, the vaccine, which was tested in a Phase III clinical trial known as the Herpevac Trial for Women, was initially found to be ineffective at protecting women against genital herpes disease caused by either herpes simplex virus type 1 (HSV-1) or herpes simplex virus type 2 (HSV-2). However, in the final analysis of the trial data, researchers found that the rate of genital herpes caused by HSV-1 was reduced by 58 percent among the volunteers who received the investigational vaccine compared with those who received the control vaccine.
The Herpevac trial, which began in 2002 and ended in 2010, was sponsored by GlaxoSmithKline (GSK) Biologicals in cooperation with the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health. Led by Robert Belshe, M.D., director of the Saint Louis University Center for Vaccine Development, the study enrolled 8,323 women ages 18 to 30 at 50 sites in the United States and Canada. At enrollment, participants were free of both HSV-1 and HSV-2. Each volunteer was randomly assigned to receive three doses of either the investigational vaccine developed by GSK or a licensed hepatitis A vaccine, which was used as a control. All participants were followed for 20 months and evaluated frequently for genital herpes disease. Blood tests determined if asymptomatic infection with HSV-1 or HSV-2 occurred in the women during the study.
In earlier research involving men and women who did not have genital herpes but whose sexual partners were known to be infected, the vaccine prevented genital herpes disease in more than 70 percent of women but had no clear effect in men. Those studies formed the basis to conduct the larger Herpevac study in women only.
Since that time, researchers have analyzed the effectiveness of the candidate vaccine against each of the two HSV subtypes. They found it to be 58 percent effective against HSV-1 genital herpes disease and 35 percent effective against HSV-1 infection, but did not observe effectiveness against HSV-2 infection. Although it is unclear why the vaccine protected against one HSV type and not the other, the researchers suspect that HSV-1 may be more susceptible to elimination by vaccine-induced antibodies than HSV-2. Additionally, the study authors write, differences between the population involved in the Herpevac trial, who reflected the general population, and those in earlier studies of the vaccine, which did not, likely contributed to differing results.
The new analysis also provides some insight into the changing roles of the viral subtypes in herpes infection. Although HSV-1 is typically associated with cold sores near the mouth and HSV-2 with genital lesions and blisters, HSV-1 has become an increasingly common cause of genital disease, an observation echoed in the NEJM analysis. Among women enrolled in the control group of the Herpevac trial, HSV-1 was found to cause more cases of genital herpes than HSV-2. Researchers will continue to examine blood samples from the Herpevac trial to better understand why the vaccine protected women from genital disease caused by HSV-1 but not HSV-2.
Both types of the herpes virus may be transmitted through sexual or other skin-to-skin contact, and also can be spread even when the infected individual shows no symptoms. In fact, most HSV infections are transmitted by asymptomatic individuals. Infants born to HSV-infected women may experience severe illness, and the virus has been identified as a risk factor for HIV transmission in adults. An estimated one in four women in the United States has genital herpes.
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RB Belshe et al. Efficacy results of a trial of a herpes simplex vaccine. New England Journal of Medicine 366(1):26-35 (2012).
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