 Two sentinel groups will enroll 3 subjects each in an open-label manner Cohorts 1 and 3 to receive VXA-RSV-f prior to enrolling either of the randomized, controlled cohorts Cohorts 2 and 4. Within the double-blinded Cohorts 2 and 4, placebo subjects will receive the same number of tablets as the vaccine subjects in that Cohort. Subjects will be enrolled and dosed in the low dose groups prior to initiation of dosing in the high dose group. 