Participants who received one of the five versions of the vaccine will be asked to take part in an optional substudy that will evaluate the safety and immunogenicity of a second vaccination 6 months after the first vaccination. In the substudy, participants will be randomly assigned to receive either the same vaccine they received in the first part of the study or a placebo vaccine. For 6 months following the second vaccination, participants will attend study visits and take part in the same study procedures that occurred in the first part of the study.