The following criteria will be reviewed on Study Days 28 and 56 following each vaccination. If any become applicable during the study, the subject will not be included in per-protocol immunogenicity evaluations, as of the exclusionary visit. The subject will, however, be encouraged to remain in the study for safety evaluations until 6 months following the last vaccination (or challenge) received. The subject will have samples obtained at the protocol-defined time-points for immunogenicity and will be included in intention-to-treat immunogenicity analysis.