A single booster immunization with the same dose of study vaccine as the primary dose (2 x 10^7 pfu/mL) will be given to those randomized at month 18 to the booster arm of the trial. However, if at any time during the observation period antibody levels fall below a predefined seroprotective threshold (yet to be defined in parallel or newly planned studies), a booster will be offered to those who have not previously received a booster injection.