Vaccination strategies for epidemic cholera in Haiti with implications for the developing world

Abstract

In October 2010, a virulent South Asian strain of El Tor cholera began to spread in Haiti. Interventions have included treatment of cases and improved sanitation. Use of cholera vaccines would likely have further reduced morbidity and mortality, but such vaccines are in short supply and little is known about effective vaccination strategies for epidemic cholera. We use a mathematical cholera transmission model to assess different vaccination strategies. With limited vaccine quantities, concentrating vaccine in high-risk areas is always most efficient. We show that targeting one million doses of vaccine to areas with high exposure to Vibrio cholerae, enough for two doses for 5% of the population, would reduce the number of cases by 11%. The same strategy with enough vaccine for 30% of the population with modest hygienic improvement could reduce cases by 55% and save 3,320 lives. For epidemic cholera, we recommend a large mobile stockpile of enough vaccine to cover 30% of a country's population to be reactively targeted to populations at high risk of exposure.

Fig. 1.

The cholera epidemic simulation model. (A) The population of Haiti in the simulation is divided into 1-km² cells. The nine departments are indicated by different colors, and rivers and highways are superimposed in blue and red, respectively. Individuals may commute to nearby locations to work or occasionally travel longer distances. (B) Each 1-km² cell is divided into communities (represented by dots) of ∼500 individuals. The river may be contaminated with V. cholerae, which can travel to downstream communities (indicated by the blue arrows). Only a limited number of communities in a cell can be in contact with the river, which is represented by the blue dots. (C) In the model's natural history of cholera, infected individuals shed V. cholerae into their communities, and susceptible individuals can be infected by this environmental source of V. cholerae. In addition, cholera is transmitted within households, which consist of 1–10 individuals. Infected residents living in a community on a river shed V. cholerae into both their community and the river.

Fig. 2.

The timing of the cholera epidemic in Haiti in 2010–2011. (A) New daily hospitalizations by department (data from ref. 3). Port-au-Prince includes the communes of Carrefour, Cité Soleil, Delmas, Kenscoff, Petion Ville, Port-au-Prince, and Tabarre. Ouest does not include Port-au-Prince. (B) Simulated new cholera cases by department. The median numbers of cases in each department from 10 stochastic simulations are plotted using the same color scheme as in A.

Fig. 3.

Simulated effects of vaccination in Haiti. (A) The simulated total cumulative case incidence when 0%, 10%, 30%, 50%, and 70% of the population is prevaccinated. The median results from 50 simulations per scenario are plotted, and the vertical lines depict the minimum and maximum values. (B) Number of newly symptomatic individuals over time in simulations using different vaccination strategies. In reactive vaccination strategies, 50,000 people are vaccinated per day starting 21 d after the epidemic begins. In the mass vaccination campaigns, 70% of individuals in randomly selected 1-km² cells are vaccinated each day. In the ring vaccination campaigns, cells are prioritized to be vaccinated 5 d after two residents of that cell become symptomatic. In the high-exposure campaigns, all cells along the rivers are prioritized to be vaccinated, regardless of the presence or absence of cholera. (C) The effectiveness of a simulated vaccination strategy is defined as the percentage of cases averted with respect to no vaccination. The points plot median estimates of the effectiveness of various strategies and amounts of vaccine using the bootstrap (50 simulation runs and 10,000 bootstrap trials), and the lines represent the 95% envelope from the bootstrap distribution. (D) The effect of adding a hygiene improvement component to reactive vaccination. Individuals in vaccinated regions may have 10% or 30% decreased exposure because of improved hygienic practices from outreach efforts.