title: Effectiveness of an oral cholera vaccine in Zanzibar: findings from a mass vaccination campaign and observational cohort study.
abstract: BACKGROUND
  Zanzibar, in east Africa, has been severely and repeatedly affected by cholera since 1978. We assessed the effectiveness of oral cholera vaccination in high-risk populations in the archipelago to estimate the indirect (herd) protection conferred by the vaccine and direct vaccine effectiveness.
  
  
  METHODS
  We offered two doses of a killed whole-cell B-subunit cholera vaccine to individuals aged 2 years and older in six rural and urban sites. To estimate vaccine direct protection, we compared the incidence of cholera between recipients and non-recipients using generalised estimating equations with the log link function while controlling for potential confounding variables. To estimate indirect effects, we used a geographic information systems approach and assessed the association between neighbourhood-level vaccine coverage and the risk for cholera in the non-vaccinated residents of that neighbourhood, after controlling for potential confounding variables. This study is registered with ClinicalTrials.gov, number NCT00709410.
  
  
  RESULTS
  Of 48,178 individuals eligible to receive the vaccine, 23,921 (50%) received two doses. Between February, 2009, and May, 2010, there was an outbreak of cholera, enabling us to assess vaccine effectiveness. The vaccine conferred 79% (95% CI 47-92) direct protection against cholera in participants who received two doses. Indirect (herd) protection was shown by a decrease in the risk for cholera of non-vaccinated residents within a household's neighbourhood as the vaccine coverage in that neighbourhood increased.
  
  
  CONCLUSIONS
  Our findings suggest that the oral cholera vaccine offers both direct and indirect (herd) protection in a sub-Saharan African setting. Mass oral cholera immunisation campaigns have the potential to provide not only protection for vaccinated individuals but also for the unvaccinated members of the community and should be strongly considered for wider use. Because this is an internationally-licensed vaccine, we could not undertake a randomised placebo-controlled trial, but the absence of vaccine effectiveness against non-cholera diarrhoea indicates that the noted protection against cholera could not be explained by bias.
  
  
  BACKGROUND
  Bill & Melinda Gates Foundation, Swedish International Development Cooperation Agency, and the South Korean Government.
authors.affiliation: Ministry of Health, Zanzibar, Tanzania.
authors.forename: Ahmed M
authors.initials: AM
authors.lastname: Khatib
authors.forename: Mohammad
authors.initials: M
authors.lastname: Ali
authors.forename: Lorenz
authors.initials: L
authors.lastname: von Seidlein
authors.forename: Deok Ryun
authors.initials: DR
authors.lastname: Kim
authors.forename: Ramadhan
authors.initials: R
authors.lastname: Hashim
authors.forename: Rita
authors.initials: R
authors.lastname: Reyburn
authors.forename: Benedikt
authors.initials: B
authors.lastname: Ley
authors.forename: Kamala
authors.initials: K
authors.lastname: Thriemer
authors.forename: Godwin
authors.initials: G
authors.lastname: Enwere
authors.forename: Raymond
authors.initials: R
authors.lastname: Hutubessy
authors.forename: Maria Teresa
authors.initials: MT
authors.lastname: Aguado
authors.forename: Marie-Paule
authors.initials: MP
authors.lastname: Kieny
authors.forename: Anna Lena
authors.initials: AL
authors.lastname: Lopez
authors.forename: Thomas F
authors.initials: TF
authors.lastname: Wierzba
authors.forename: Said Mohammed
authors.initials: SM
authors.lastname: Ali
authors.forename: Abdul A
authors.initials: AA
authors.lastname: Saleh
authors.forename: Asish K
authors.initials: AK
authors.lastname: Mukhopadhyay
authors.forename: John
authors.initials: J
authors.lastname: Clemens
authors.forename: Mohamed Saleh
authors.initials: MS
authors.lastname: Jiddawi
authors.forename: Jacqueline
authors.initials: J
authors.lastname: Deen
mesh_terms: D000284:Administration, Oral; D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D002771:Cholera / Q000453:epidemiology* / Q000517:prevention & control*; D022121:Cholera Vaccines / Q000008:administration & dosage* / Q000276:immunology*; D015331:Cohort Studies; D005260:Female; D006801:Humans; D032521:Immunity, Herd; D015994:Incidence; D008297:Male; D032541:Mass Vaccination / Q000379:methods*; D008875:Middle Aged; D013636:Tanzania / Q000453:epidemiology; D016896:Treatment Outcome; D015164:Vaccines, Inactivated / Q000008:administration & dosage / Q000276:immunology; D055815:Young Adult
chemical_list: D022121:Cholera Vaccines; D015164:Vaccines, Inactivated