Statistical Analysis

The trial was designed to have 80% power to detect a vaccine efficacy of 75% with 45% as the lower limit of the 95% confidence interval. It met the information goal, observing 70 of a planned 72 cases of genital herpes disease in the per-protocol cohort. The trial was monitored by an independent data safety and monitoring board sponsored by the National Institute of Allergy and Infectious Diseases, which met quarterly and reviewed the study for safety. At a prespecified interim analysis, the board also reviewed the trial for futility. The sample size was extended once in response to higher-than-anticipated attrition. Vaccine efficacy was estimated as 1 minus the relative risk from a Cox proportional-hazards model fit to the time to first acquisition of each study end point. Rates of loss-to-follow-up were similar between the two study groups, and noninformative censoring was assumed. A post hoc assessment of demographic and behavioral risk factors for HSV acquisition was performed with the use of a Cox proportional-hazards model adjusted for the receipt of HSV vaccine. All reported P values are two-tailed and have not been adjusted for multiple testing. The per-protocol and intention-to-treat cohorts are defined in the legend for Figure 1.
Results
Characteristics of the Study Population

Fifty clinical sites in the United States and Canada screened a total of 31,770 women for antibodies to HSV-1 and HSV-2; 12,468 women were seronegative for both HSV-1 and HSV-2, of whom 8323 met the other eligibility criteria and were enrolled between January 14, 2003, and November 19, 2007.

Vaccine Efficacy

In the control group, HSV-1 was a more common cause of genital disease than HSV-2 (21 cases caused by HSV-1 vs. 14 cases caused by HSV-2). Efficacy against genital disease caused by HSV-1 was observed (vaccine efficacy, 58%; 95% CI, 12 to 80) (Figure 2B), but efficacy was not observed against HSV-2 disease (−38%; 95% CI, −167 to 29) (Figure 2C). Three doses of vaccine were associated with efficacy against HSV-1 (77%; 95% CI, 31 to 92) but not HSV-2 (−40%; 95% CI, −234 to 41). An analysis in which the case definition was limited to culture-positive cases (excluding HSV cases diagnosed according to clinical and serologic criteria) also showed efficacy against HSV-1 (two-dose efficacy, 69%; 95% CI, 25 to 87; three-dose efficacy, 82%; 95% CI, 35 to 95).

The HSV vaccine provided protection against infection caused by HSV-1 or HSV-2 (efficacy, 22%; 95% CI, 2 to 38). This overall finding of protection against infection was driven by efficacy against HSV-1 infection (35%; 95% CI, 13 to 52), whereas efficacy against HSV-2 infection was not observed (−8%; 95% CI, −59 to 26).