This site needs JavaScript to work properly. Please enable it to take advantage of the complete set of features!
Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

NIH NLM Logo
Log in
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2007 Nov 27;4(11):e336.
doi: 10.1371/journal.pmed.0040336.

Controlling endemic cholera with oral vaccines

Affiliations

Controlling endemic cholera with oral vaccines

Ira M Longini Jr et al. PLoS Med. .

Abstract

Background: Although advances in rehydration therapy have made cholera a treatable disease with low case-fatality in settings with appropriate medical care, cholera continues to impose considerable mortality in the world's most impoverished populations. Internationally licensed, killed whole-cell based oral cholera vaccines (OCVs) have been available for over a decade, but have not been used for the control of cholera. Recently, these vaccines were shown to confer significant levels of herd protection, suggesting that the protective potential of these vaccines has been underestimated and that these vaccines may be highly effective in cholera control when deployed in mass immunization programs. We used a large-scale stochastic simulation model to investigate the possibility of controlling endemic cholera with OCVs.

Methods and findings: We construct a large-scale, stochastic cholera transmission model of Matlab, Bangladesh. We find that cholera transmission could be controlled in endemic areas with 50% coverage with OCVs. At this level of coverage, the model predicts that there would be an 89% (95% confidence interval [CI] 72%-98%) reduction in cholera cases among the unvaccinated, and a 93% (95% CI 82%-99%) reduction overall in the entire population. Even a more modest coverage of 30% would result in a 76% (95% CI 44%-95%) reduction in cholera incidence for the population area covered. For populations that have less natural immunity than the population of Matlab, 70% coverage would probably be necessary for cholera control, i.e., an annual incidence rate of < or = 1 case per 1,000 people in the population.

Conclusions: Endemic cholera could be reduced to an annual incidence rate of < or = 1 case per 1,000 people in endemic areas with biennial vaccination with OCVs if coverage could reach 50%-70% depending on the level of prior immunity in the population. These vaccination efforts could be targeted with careful use of ecological data.

PubMed Disclaimer

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Basic Model Parameter Distributions
Modeled natural history of cholera [–10]. People start out partially susceptible. Newly infected people pass through the incubating state (mean 3.6 d) and infectious state (mean 10.5 d) after which they recover with partial immunity or die. The probability distributions of the incubation and infectious periods are shown in the figure. We assume that 10% of infected people develop overt cholera symptoms and 90% will be asymptomatic. We further assume that symptomatic people are ten times as infectious as asymptomatics. Additionally, this model allows for 75% of symptomatic working males to withdraw to their subregion [Pr(withdrawal after ill)].
Figure 2
Figure 2. Schematic of Effectiveness Comparisons for Two Subregions
Subregion 1 has a fraction f 1 > 0 people vaccinated, while the comparison subregion 2 has nobody vaccinated, i.e., f 2 = 0. We let rij be the cholera infection rate for people in subregion j with vaccination status i, where i = 0 for unvaccinated and i = 1 for vaccinated. The indirect effect of vaccination is measured by comparing the infection rates between the unvaccinated in the two subregions. Thus, the indirect vaccine effectiveness, i.e., IVEF, when comparing subregion 1 to 2 is IVEF12 = 1 − (r 01/r 02). The overall effect of vaccination is measured by comparing the average (over the vaccinated and unvaccinated groups) infection rates between the two subregions. Thus, the overall vaccine effectiveness, i.e., OVEF, is OVEF12 = 1 − (r. 1/r. 2), where the · indicates averaging over the vaccinated and unvaccinated. The total effect of vaccination is measured by comparing the infection rate in the vaccinated in subregion 1 to the unvaccinated in subregion 2. Thus, the total vaccine effectiveness, i.e., TVEF, is TVEF12 = 1 − (r 11/r 02). The direct effect of vaccination is measured by comparing the infection rates in the vaccinated and unvaccinated in the same subregion. The direct vaccine effectiveness, i.e., DVEF, is DVEF1 = 1 − (r 11/r 01). In general, these effectiveness measures could be computed across any gradient of coverage, |f 1 − f 2|, other than those with f 2 = 0.
Figure 3
Figure 3. Simulated Number of Cholera Cases per 1,000 over a 180-Day Period in the Matlab Study Population for a Single Stochastic Realization
(A) No vaccination. (B) 14% vaccination coverage of women and children. (C) 38% vaccination coverage. (D) 58% vaccination coverage.
Figure 4
Figure 4. Average Overall Vaccine Effectiveness When Varying Relative Susceptibility
These comparisons are for relative susceptibility in populations ranging from 1.5 to 2.5 times as susceptible as Matlab. The Matlab results are shown when the multiplier is 1. For populations that are 2–2.5 times as susceptible as Matlab, at least 70% vaccine coverage is needed to achieve an overall effectiveness of at least 80%. The vaccine efficacies are preset at VES = 0.7 and VEI = 0.5.

References

    1. WHO. Cholera, 2005. Wkly Epidemiol Rec. (2006);81:297–308. - PubMed
    1. Clemens JD, Sack DA, Harris JR, van Loon F, Chakraborty J, et al. Field trial of oral cholera vaccines in Bangladesh: results from three-year follow-up. Lancet. 1990;335:270–273. - PubMed
    1. Durham LK, Longini IM, Halloran ME, Clemens JD, Nizam A, et al. Estimation of vaccine efficacy in the presence of waning: application to cholera vaccines. Am J Epidemiol. 1998;147:948–959. - PubMed
    1. Thiem VD, Hossain MM, Son ND, Hoa NT, Rao MR, et al. Coverage and costs of mass immunization of an oral cholera vaccine in Vietnam. J Health Popul Nutr. 2003;21:304–308. - PubMed
    1. WHO. Cholera, 2005. Wkly Epidemiol Rec. (2006);80:261–268. - PubMed

Publication types

Full text links
Public Library of Science full text link Public Library of Science Free PMC article
Cite

AltStyle によって変換されたページ (->オリジナル) /