Participants will be assigned to one of two treatment groups: one will receive the experimental vaccine; the other will receive a control substance (inactive salt solution). The first five people assigned to the vaccine group will receive the lowest study dose of the vaccine. If this dose is safe, it will be increased three times for the next group of five and then eight times for the last group of five. Each group will have a total of seven people - five will receive the vaccine and two will get the salt solution.