Herd protection of unvaccinated adults by oral cholera vaccines in rural Bangladesh.

K Zaman,Mohammad Ali,Peter Kim,John Clemens

doi:10.1093/inthealth/ihy085

Abstract

BackgroundPast research has suggested that the most cost-effective approach to using oral cholera vaccines (OCVs) to control endemic cholera may be to target only children <15 y of age. However, the assumption that vaccination of children with OCVs protects unvaccinated adults has never been tested.MethodsWe reanalyzed the data of an OCV trial in Bangladesh in which children 2–15 y of age and women >15 y of age were allocated to OCV or placebo and assessed herd protection by relating the risk of cholera in each nonvaccinated adult (>15 y) to OCV coverage (OCVC) of residents residing in virtual clusters within 500 m of the residence of that unvaccinated adult.ResultsThe risk of cholera in unvaccinated adults decreased by 14% with each 10% increase of OCVC of all targeted age groups (95% 7 to 21%, p=0.0004). Also, the risk of cholera in unvaccinated adults decreased by 13% with each 10% increase in OCVC of children 2–15 y of age (95% CI 6 to 20%, p=0.0007). A high correlation between levels of OCVC of children and adult females precluded an assessment of the herd protection of unvaccinated adults by vaccinating children <16 y of age, independent of concomitant vaccination of adult women.ConclusionsUnvaccinated adults benefitted from herd protection conferred by OCVs in this trial. Vaccination of children may be sufficient to confer this protection, but this possibility needs to be evaluated in further studies.

Highlights

Inactivated oral cholera vaccines (OCVs) are recommended by the World Health Organization (WHO) to help control cholera in both epidemic and endemic settings in conjunction with other cholera prevention and control strategies, such as clean water and sanitation.[1]
We considered cholera episodes occurring during the period 1 June 1985–31 May 1986, an interval in which OCV herd protection was demonstrable in our earlier analyses,[9] and following our previous analyses, we defined a person as vaccinated if she/he received at least two doses of either OCV, which, because of the similarity of their constituents and protective efficacy, were combined for this analysis
We first examined the association between the risk of cholera in unvaccinated persons and approximate quintiles of OCV coverage (OCVC) of the surrounding age-targeted population

Summary

Introduction

Inactivated oral cholera vaccines (OCVs) are recommended by the World Health Organization (WHO) to help control cholera in both epidemic and endemic settings in conjunction with other cholera prevention and control strategies, such as clean water and sanitation.[1] To facilitate the use of these vaccines in such settings, since 2013, Gavi, the Vaccine Alliance, has supported a global OCV stockpile, with the WHO as the stockpile secretariat.[2] The number of doses of OCV deployed from the stockpile has increased almost exponentially with each year since the stockpile’s creation. The assumption that vaccination of children with OCVs protects unvaccinated adults has never been tested

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