The study consists of a Screening Period and a Treatment Period. During the Screening Period (Days -30 to -1), informed consent is obtained, eligibility for study entry is assessed and baseline tests are performed. Subjects who satisfy all inclusion/exclusion criteria are eligible to enter the Treatment Period and are randomized to one of the two treatment groups. On Day 0 subjects will receive either YF-VAX® plus saline, or YF-VAX® plus Gama STAN S/D®. Subjects return to the clinic for daily visits on day 1, 2, 3, 5, 7, 9, 11, 14, 30, and 91. The total duration of participation for an individual subject is approximately 91 days.