Herd protection by a bivalent killed whole-cell oral cholera vaccine in the slums of Kolkata, India.
BACKGROUND
We evaluated the herd protection conferred by an oral cholera vaccine using 2 approaches: cluster design and geographic information system (GIS) design.


METHODS
Residents living in 3933 dwellings (clusters) in Kolkata, India, were cluster-randomized to receive either cholera vaccine or oral placebo. Nonpregnant residents aged≥1 year were invited to participate in the trial. Only the first episode of cholera detected for a subject between 14 and 1095 days after a second dose was considered. In the cluster design, indirect protection was assessed by comparing the incidence of cholera among nonparticipants in vaccine clusters vs those in placebo clusters. In the GIS analysis, herd protection was assessed by evaluating association between vaccine coverage among the population residing within 250 m of the household and the occurrence of cholera in that population.


RESULTS
Among 107 347 eligible residents, 66 990 received 2 doses of either cholera vaccine or placebo. In the cluster design, the 3-year data showed significant total protection (66% protection, 95% confidence interval [CI], 50%-78%, P<.01) but no evidence of indirect protection. With the GIS approach, the risk of cholera among placebo recipients was inversely related to neighborhood-level vaccine coverage, and the trend was highly significant (P<.01). This relationship held in multivariable models that also controlled for potentially confounding demographic variables (hazard ratio, 0.94 [95% CI, .90-.98]; P<.01).


CONCLUSIONS
Indirect protection was evident in analyses using the GIS approach but not the cluster design approach, likely owing to considerable transmission of cholera between clusters, which would vitiate herd protection in the cluster analyses.


BACKGROUND
NCT00289224.
International Vaccine Institute, Seoul, Republic of Korea. mali@ivi.int
Mohammad
M
Ali
Dipika
D
Sur
Young Ae
YA
You
Suman
S
Kanungo
Binod
B
Sah
Byomkesh
B
Manna
Mahesh
M
Puri
Thomas F
TF
Wierzba
Allan
A
Donner
G Balakrish
GB
Nair
Sujit K
SK
Bhattacharya
Mandeep Singh
MS
Dhingra
Jacqueline L
JL
Deen
Anna Lena
AL
Lopez
John
J
Clemens
D000284:Administration, Oral; D000293:Adolescent; D000328:Adult; D002648:Child; D002675:Child, Preschool; D002771:Cholera / Q000276:immunology / Q000517:prevention & control*; D022121:Cholera Vaccines / Q000008:administration & dosage / Q000276:immunology*; D016000:Cluster Analysis; D006801:Humans; D032521:Immunity, Herd; D007194:India; D007223:Infant; D011204:Poverty Areas; D016016:Proportional Hazards Models; D012306:Risk; D016896:Treatment Outcome; D014505:Urban Population; D014611:Vaccination; D015164:Vaccines, Inactivated / Q000008:administration & dosage / Q000276:immunology; D055815:Young Adult
D022121:Cholera Vaccines; D015164:Vaccines, Inactivated