Preliminary data on correlates of protection against HSV-1 are available. Antibody, but not cellular immunity, was correlated with protection against HSV-1. Among the HSV vaccinees tested in the immunogenicity cohort, 8 were subsequently infected by HSV-1 and 10 by HSV-2. Subjects with HSV-1 infection had significantly lower gD-2 ELISA antibody titers at month 7 (mean titer, 3561) than subjects who remained uninfected (mean titer, 6875; P=0.04). This was not the case for subjects with HSV-2 infection (mean titer, 6339; P=0.78). Cellular immune responses at month 2 or month 7 did not differ significantly between subjects who subsequently became infected with HSV-1 or HSV-2 and vaccinees who remained uninfected (see the Supplementary Appendix for details).

The efficacy of the gD-2 candidate vaccine against HSV-1 is important because epidemiologic studies suggest that sexual transmission of HSV-1 is increasing in the United States,1 although its prevalence among U.S. children is decreasing.11 Among control subjects in the present study, 60% of the cases of genital disease and two thirds of the infections (with or without disease) were caused by HSV-1. These data are similar to the finding that HSV-1 is the most common cause of genital herpes in college students and young heterosexual women, and similar trends are reported in other countries. HSV-1 now rivals HSV-2 as a cause of neonatal herpes disease. Public health officials and researchers will need to closely monitor seroprevalence trends as future herpes vaccines are developed and assessed.

Although the development of a vaccine that provides protection against HSV-1 genital disease is a substantial step forward, additional progress is needed before a herpes vaccine is likely to be approved for general use. Any candidate vaccine will probably have to have proven efficacy against both HSV-1 and HSV-2 disease. Prevention of infection (with or without disease) is also an important goal, because asymptomatic genital shedding of HSV may lead to the spread of the virus to newborns or sexual partners. This is a more difficult goal to achieve and may require approaches such as live attenuated vaccines or vaccine vectors to generate protection.