Prolonging herd immunity to cholera via vaccination: Accounting for human mobility and waning vaccine effects

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Abstract
Oral cholera vaccination is an approach to preventing outbreaks in at-risk settings and controlling cholera in endemic settings. However, vaccine-derived herd immunity may be short-lived due to interactions between human mobility and imperfect or waning vaccine efficacy. As the supply and utilization of oral cholera vaccines grows, critical questions related to herd immunity are emerging, including: who should be targeted; when should revaccination be performed; and why have cholera outbreaks occurred in recently vaccinated populations? We use mathematical models to simulate routine and mass oral cholera vaccination in populations with varying degrees of migration, transmission intensity, and vaccine coverage. We show that migration and waning vaccine efficacy strongly influence the duration of herd immunity while birth and death rates have relatively minimal impacts. As compared to either periodic mass vaccination or routine vaccination alone, a community could be protected longer by a blended “Mass and Maintain” strategy. We show that vaccination may be best targeted at populations with intermediate degrees of mobility as compared to communities with very high or very low population turnover. Using a case study of an internally displaced person camp in South Sudan which underwent high-coverage mass vaccination in 2014 and 2015, we show that waning vaccine direct effects and high population turnover rendered the camp over 80% susceptible at the time of the cholera outbreak beginning in October 2016. Oral cholera vaccines can be powerful tools for quickly protecting a population for a period of time that depends critically on vaccine coverage, vaccine efficacy over time, and the rate of population turnover through human mobility. Due to waning herd immunity, epidemics in vaccinated communities are possible but become less likely through complementary interventions or data-driven revaccination strategies. Cholera vaccination can be a relatively quick means to temporarily prevent cholera from spreading in an at-risk population. In order to understand how long this temporary protection remains and therefore the timeline for when we need to install longer-term water and sanitation solutions, we must know how long we can expect the vaccine to provide herd protection. To answer this and other related questions, we developed a mathematical model to test different vaccination strategies in a simulated population and in a case study of a displaced-persons camp in Bentiu, South Sudan. We found that the duration of vaccine-derived herd protection can be short (<1 year) in settings of moderate transmission potential and high population mobility, but this duration can be extended through a strategy that complements a one-time mass vaccination campaign with ongoing, routine vaccination. We show that short-lived vaccine efficacy and high population turnover in the Bentiu camp can help explain why the camp had a cholera outbreak despite two high-coverage vaccination campaigns in the two previous years. Our results support, and provide timelines for, cholera vaccination as initial protection while longer-term structural interventions can be implemented.
Funding Information
  • National Institutes of Health (T32AI007535-16A1)
  • Bill and Melinda Gates Foundation (OPP153556)
  • National Institute of General Medical Sciences (U54GM088558)
  • Bill and Melinda Gates Foundation (OPP1089243)

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