A randomized double-blind trial to evaluate the safety of a novel recombinant virus, ICP10ΔPK, for reduction or prevention of recurrent herpes simplex virus type 2 (HSV-2) infection was carried out in public hospitals in Mexico City. Persons having a minimum of 5 documented herpetic recurrences in the previous year were randomized for vaccination. Patients were examined within 72 hours of lesion occurrence. If accepted into the study, the patient was inoculated subcutaneously in the upper deltoid muscle area at days 7, 17, and 28 after initiation of lesion occurrence. Recurrences were recorded by patient diary and physician examination. During the observation period (extending from 10 to 180 days after the last booster dose), recurrences in the vaccine (V) group were prevented completely in 37.5% of the patients, whereas in the placebo (P) group, 100% of the patients had at least one recurrence (P=.068). Vaccinated patients had fewer recurrences (V, 1.58; P, 3.13 [P=.028]). The mean number of illness days was 10 for the vaccine group and 18 for the placebo group (P=.028). Further studies to evaluate this vaccine and its dosimetry for the treatment of genital herpes infections appear warranted.