Most cases of genital herpes are due to herpes simplex virus (HSV)-2. The rate of HSV-2 infections increased by 30% from 1988 to 1994. In addition to genital herpes, transmission of HSV-2 to neonates causes severe life-threatening infections. Recent studies suggest a link between genital herpes and increased rates of transmission of HIV and elevated levels of HIV in the blood. Therefore, an effective HSV-2 vaccine is needed.[unreadable] [unreadable] Several HSV-2 vaccines have tested in humans for prevention or reduction genital herpes disease. A vaccine containing a single viral protein (HSV-2 glycoprotein D) reduced the rate of HSV-2 disease in women who were not previously infected with HSV-1 in two randomized, controlled clinical trials. The HSV-2 glycoprotein D vaccine, however, showed no efficacy in women previously infected with HSV-1 or in men. We postulated that the limited efficacy of the HSV-2 glycoprotein D vaccine is likely due to inadequate induction of cellular immune responses. [unreadable] [unreadable] We have been evaluating a candidate HSV-2 vaccine deleted for two essential genes, termed HSV-2 dl5-29, which was developed by David Knipe at Harvard University. Previously, we compared the cellular immune response of HSV-2 dl5-29 to glycoprotein D in mice and found that HSV-2 dl5-29 resulted in significantly higher systemic HSV-2 specific lymphocytes than glycoprotein D, and more rapid accumulation of HSV-2 specific lymphocytes in trigeminal ganglia after challenge with wild-type virus compared to HSV-2 glycoprotein D. Guinea pigs vaccinated with HSV-2 dl5-29 had higher levels of virus neutralizing antibodies in the sera compared with animals inoculated with recombinant HSV-2 glycoprotein D. Since the cellular immune and neutralizing antibody responses to HSV-2 dl5-29 were superior to that seen with recombinant HSV-2 glycoprotein D, HSV-2 dl5-29 is an attractive candidate for early phase clinical trials in humans.[unreadable] [unreadable] We are currently testing a derivative of HSV-2 dl5-29 in which an additional HSV-1 gene has been deleted from this virus. The deleted gene (the viral host shut-off protein) normally impairs the immune response to the virus; therefore animals infected with the deletion mutant could have improved immune responses and protection after challenge with wild-type virus. The HSV-2 dl5-29 derivative (dl5-29-41L) was reported to be more immunogenic and more effective than HSV-2 dl5-29 in protecting mice from challenge with HSV-2.[unreadable] [unreadable] Since some immunocompromised persons could be inadvertently vaccinated with HSV-2 dl5-29 if the vaccine is used, we tested the vaccine for its safety in immunocompromised mice. We found that HSV-2 dl5-29 was at least 250,000-fold less virulent than parental virus by intracranial inoculation in healthy mice, and caused no disease after subcutaneous injection in SCID mice. Thus, HSV-2 dl5-29 was safe in immunocompromised animals.[unreadable] [unreadable] We compared the immunogenicity and efficacy of HSV-2 dl5-29-41L with dl5-29 in guinea pig and mouse models of genital herpes. Vaccination of guinea pigs with dl5-29 or its derivative (dl5-29-41L) resulted in similar levels of neutralizing antibody titers against HSV-2. When vaccinated guinea pigs were challenged with HSV-2, animals receiving either vaccine shed similar titers of HSV-2, had similar disease severity scores, had similar numbers of genital herpes recurrences during the first 3 months after infection, and had similar levels of HSV-2 DNA in their ganglia. Both vaccines completely protected mice from lethal challenge with HSV-2 and similar levels of induced virus-specific cellular immune responses. Therefore both vaccines were equally protective in mice and guinea pigs, and dl5-29 is very safe in immunocompromised animals.