Participants will be randomly assigned to receive either the experimental vaccine or a placebo (a salt solution that does not contain any active substance) and will be divided into three groups, based on their entry into the study. Of the first seven people enrolled (Group 1), five will receive a 2-mg dose of vaccine and two will receive placebo. If the vaccine is safe at this dose, then in Group 2, five people will receive a 4-mg dose of vaccine and two will receive placebo. If this dose is safe, then in Group 3, thirty people will receive an 8-mg dose of vaccine and six will receive placebo.