The proposed study will determine if giving the second dose of the OCV using a longer interval will result in a response to the second dose that is not inferior, or perhaps even results in a more prolonged elevated vibriocidal titre. From an immunological standpoint, there could be advantages to a longer dose interval if this resulted in a true booster response which sustained high titre of antibody. While acknowledging that the vibriocidal titer is not an established correlate of protection, it is the best correlate of the immune response following vaccination. It seems logical that a sustained high titer is likely to be more effective. Unfortunately, the clinical trials comparing the 2 week and 4 week interval with Shanchol only examined the serum titers shortly after the dosing (about 2 weeks). Follow-up serum samples were not obtained to discern how these dose intervals compared in terms of duration of elevated serum titres.