The sample size and power are based upon a fixed number of events design assuming equal incidence rates in both the IG and vaccine populations and a one-sided significance level of =0.05, and were calculated under the following testing hypotheses: Ho: PV / PIG 3.0 versus Ha: PV / PIG <3.0. This is equivalent to testing that the upper bound of the 90% confidence interval on the observed relative risk is <3.0. For a fixed accrual of 44 evaluable secondary cases of confirmed hepatitis A, the study has 95% power to rule out a relative risk of 3.0 or greater. If the total number of cases observed in this study is 44 and if no more than 27 (i.e., 61.4%) of the cases are in the vaccine group, there will be significant evidence to reject the above null hypothesis. If the total number of cases is larger than 44, the maximum number of cases in the vaccine group that can be observed and still declare success will increase accordingly.