Various field studies has found that the modified , bivalent, whole cell - based oral cholera vaccine (OCV) to be safe, immunogenic and effective with protective efficacy of 67 % in earlier clinical trials. However, the effectiveness of the vaccine in "real" life situation using the public health system is unknown. It is critical to follow up in the same population, where pilot introduction of OCV was introduced and evaluate vaccine proactive effectiveness at individual as well as at population level. The follow - up and determination of effectiveness of mass OCV vaccination was requested by State Government.

The overall goal of this study is to evaluate the protective effectiveness of one or two doses of modified, bivalent, killed whole cell based OCV, given at least 14 days apart, when delivered through community - based mass vaccination campaign using existing public health infrastructure in a high - risk population in Satyabadi block of Puri district, Orissa, India.

This study has following objectives

Primary objectives:

* To evaluate the individual level protective effectiveness of one or two doses of OCV against culture confirmed cholera episodes, severe enough to seek a formal health care.

Secondary objectives:

• To evaluate population - level effectiveness (herd effects)of OCV delivered through a community based mass vaccination when the vaccine is delivered to more than half of population at risk.
• To determine inverse correlation between vaccine coverage and cholera incidence among diverse geographical clusters.

Inclusion criteria for cases of the main case-control study are as follows:

• Giving verbal informed consent/assent, or in the case of minors, a parent or guardian give informed consent to participate in the study
• Living in the study area since the start of the mass vaccination
• Submitted a faecal specimen
• Whose residence could be located
• Whose stool specimens yield V. cholera O1 or O139
• Belonging to study population through census database

Exclusion criteria:

• Not giving verbal informed consent/assent, or in the case of minors, a parent or guardian does not give informed consent to participate in the study
• Not living in the study area since the start of the mass vaccination
• No faecal specimen
• Whose residence could not be located
• Whose stool specimens does not yield V. cholera O1 or O139
• Not belonging to study population through census database

The same population of Satyabadi block, Puri, Orissa, where Pilot introduction of Oral Cholera Vaccine (OCV) was conducted during May - June, 2011. During this campaign, the OCV vaccine was delivered through community based mass vaccination utilizing public health system.