Solicited reports of adverse events included redness, swelling, and pain at the injection site, as well as fatigue, fever, headache, and malaise. The HSV vaccine was more reactogenic and was associated with local pain, redness, and swelling more often than was the control vaccine. There was a small but significant increase in systemic symptoms, including fatigue, fever, headache, and malaise, in the HSV-vaccine group (Table 3). Dose 2 and dose 3 of the HSV vaccine were not associated with increased reports of adverse events; in contrast, reactogenicity decreased slightly with additional vaccination.


The HSV vaccine was immunogenic and stimulated ELISA and neutralizing antibodies. As expected, control subjects did not have antibody to gD-2 on ELISA or neutralization of HSV-2. Geometric mean gD-2 ELISA titers were 21 at baseline and 6809 at month 7 after three doses of HSV vaccine; ELISA titers waned to 769 by month 20. HSV-2 neutralizing antibodies developed after two doses of HSV vaccine, but the median value fell to an undetectable level by study month 6 (see the Supplementary Appendix for details). After dose 3, HSV-2 neutralizing antibodies were again above the limit of detection (the mean titer at month 7 was 29) but fell to a median value that was undetectable by study month 16.


Our findings of vaccine efficacy against HSV-1 and lack of efficacy against HSV-2 are puzzling in view of the previous two studies involving discordant couples that showed efficacy of this gD-2 vaccine against HSV-2.4 The difference in efficacy is likely to be due to some factor in the two populations studied. The distinguishing feature of discordant couples is that they are a highly selected group in which the uninfected partner is potentially repeatedly exposed to HSV by the infected partner. Attack rates of HSV-2 genital disease in the prior studies of gD-2 vaccine were high among uninfected women in discordant couples (13.9% for 19 months or 8.4% per year) and were reduced significantly by the vaccine (efficacy, 73% and 74% in the two trials; P=0.01 and P=0.02, respectively).4 Too few cases of HSV-1 genital disease occurred in women in the two previous studies to assess efficacy against HSV-1 (one case in each study among women who were seronegative at study entry).4 Potential reasons for vaccine efficacy in the discordant-couple population include selection bias for a population of women with relative resistance to HSV-2, with added benefits from a subunit vaccine; an undefined immunologic priming event from chronic sexual exposure to HSV-2 viral antigens from the infected partner; and less frequent sexual activity due to the long-term nature of the relationship as compared with sexual activity by couples in new relationships.

It is not apparent why the biologic characteristics of HSV-1 are different from those of HSV-2; the gD-2 vaccine induces significant protection against genital HSV-1 disease as well as HSV-1 infection, but not against disease or infection caused by HSV-2. Genital HSV-1 may be acquired primarily through oral–genital sex (although a history of oral sex was not a risk factor for HSV-1 acquisition in our study); a lower inoculum, an oral–genital route (possibly less traumatic sex), and a less suitable environment for HSV-1 replication are all possible explanations for protection by the vaccine against HSV-1 but not HSV-2. The gD-2 antigen is derived from HSV-2, but 89% amino acid homology is shared with gD-2 from HSV-1, which may explain the protection against HSV-1. Type-specific immune responses to vaccine antigen may reveal differences in antibody activity against HSV-1 and HSV-2. Whether HSV-1 is more easily neutralized by vaccine-induced antibodies must be determined by further laboratory studies.