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

Abstract

Background Past 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. Methods We 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. Results The 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. Conclusions Unvaccinated 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.

Diagnosis, Management, and Future Control of Cholera

“…During pregnancy, severe dehydration can lead to premature delivery, miscarriage, and fetal death. OCVs were found to be safe, with no adverse fetal outcomes observed in several studies (135)(136)(137)(138). The WHO now recommends the use of OCVs for pregnant mothers in areas where cholera is endemic to prevent severe dehydrating cholera that may harm the fetus.…”

“…It has been shown that inactivated OCVs can give significant herd protection in various study settings, which were analyzed using geographic information system (GIS) tools (138). The herd-protective effects of OCVs were measured using various study designs (individually or cluster randomized trials and cohort or case-control studies), and significant herd protection (both direct and indirect) against cholera was seen among unvaccinated persons and in the community (138). Mathematical modeling of cholera transmission (139) using Matlab showed that 93% of cholera infections in Bangladesh can be prevented with 50% OCV coverage (140).…”

“…A large feasibility trial in Bangladesh revealed that children less than 3 years of age had a 47% reduction in the incidence of cholera if their mothers were given OCVs (139). Another analysis by Ali et al showed that the chance of having cholera in the unvaccinated adult population was reduced by 14% with a 10% rise in OCV coverage in all age strata with a killed oral vaccine (138).…”

Combating Cholera

“…Although IgA has limited systemic circulation (~6 months), the memory B cells that are responsible for preventing cholera infection persist and can quickly expand and differentiate into plasmablasts and eventually the plasma cells, which can reseed protective antibodies upon antigen-contact activation 28 . Moreover, OCVs could provide herd immunity to unvaccinated adults, but the effect in unvaccinated children requires further study 29 .…”

The Impact of Improved Water Supply on Cholera and Diarrhoeal Diseases in Uvira, Democratic Republic of the Congo: A Protocol for a Pragmatic Stepped-Wedge Cluster Randomised Trial and Economic Evaluation

“…The second axis, vaccination campaigns, is supported by the availability of oral cholera vaccines (OCV) which can provide protection against cholera for two to four years [26,27] and indirect protection via herd immunity has also been documented [28][29][30][31]. A global stockpile was created in 2013 to ensure OCV availability for cholera control in outbreaks and humanitarian emergencies [32].…”