10. Day 28: (± 3 days) Clinical evaluation and Study Termination Sample Size Calculation and Outcome (Primary and Secondary) Variable(s) Safety and Immunogenicity study of the Shantha cholera vaccine: The ''non-inferiority'' approach using a 1 -sided 95% confidence interval was used to calculate the sample size since this allowed us to rule out clinically unacceptable high rates of diarrheal adverse event occurring during the 3 days after either dose as well as establish that the vaccine induced adequate seroconversion to V. cholerae O1 among recipients. Assuming a 10% diarrheal rate among placebo and vaccine recipients alike, to exclude a vaccine-placebo difference in the rate of diarrhea of greater than 20% (upper boundary of the 1-tailed 95% confidence interval) with a power of 0.9, the minimum number of subjects required for each group was in each group for both adverse event was 46. The sample size is calculated with the assumptions that it is important to evaluate whether the vaccine induces acceptable serum vibriocidal responses in relation to the placebo group. Based on an immunogenicity study of the whole-cell killed oral cholera vaccine in Bangladesh, we make the following assumptions. For serum vibriocidal responses, defined as >4-fold increases between baseline and post-second dose in either Inaba or Ogawa antibodies, for each age group (12-23 months, 2-5 years and 18-45 years), we assume 1) the background rate of responses in the placebo group will be 5% after the second dose and 2) the true rate of vibriocidal responses in the vaccine groups is 25%. At p <0.05 (1-tailed), 0.8 power, and a 1 to 1 allocation of vaccine and placebo, a total of 46 subjects per group would be needed. Thus for safety and immunological responses the number is 46 per group. With 15% attrition rate, it is 53. We will select 55 in each group (110 per age group for both vaccine and placebo recipient).