A sample size for efficacy of 80 participants including 40 subjects in the group receiving YF-VAX® plus saline and 40 subjects receiving YF-VAX® plus Gama STAN S/D® provides 80% power in a one-sided test at significance level alpha of 0.05, assuming 93% of subjects not receiving immune globulin will exhibit detectable viremia and a reduction of 35% (to 61% of subjects) following administration of immune globulin.