Abstract
Reactive vaccination has recently been adopted as an outbreak response tool for cholera and other infectious diseases. Owing to the global shortage of oral cholera vaccine, health officials must quickly decide who and where to distribute limited vaccine. Targeted vaccination in transmission hotspots (i.e. areas with high transmission efficiency) may be a potential approach to efficiently allocate vaccine, however its effectiveness will likely be context-dependent. We compared strategies for allocating vaccine across multiple areas with heterogeneous transmission efficiency. We constructed metapopulation models of a cholera-like disease and compared simulated epidemics where: vaccine is targeted at areas of high or low transmission efficiency, where vaccine is distributed across the population, and where no vaccine is used. We find that connectivity between populations, transmission efficiency, vaccination timing and the amount of vaccine available all shape the performance of different allocation strategies. In highly connected settings (e.g. cities) when vaccinating early in the epidemic, targeting limited vaccine at transmission hotspots is often optimal. Once vaccination is delayed, targeting the hotspot is rarely optimal, and strategies that either spread vaccine between areas or those targeted at non-hotspots will avert more cases. Although hotspots may be an intuitive outbreak control target, we show that, in many situations, the hotspot-epidemic proceeds so fast that hotspot-targeted reactive vaccination will prevent relatively few cases, and vaccination shared across areas where transmission can be sustained is often best.
Highlights
Reactive vaccination has become an important tool in the fight against diseases such as measles [1], meningitis [2], foot-and-mouth disease [3] and cholera [4]
To ensure that a minimal supply of oral cholera vaccines (OCVs) is available for rapid deployment, a stockpile large enough to vaccinate one million individuals was established by the World Health Organization (WHO)
The populations are considered to be fully susceptible to the disease but we explore the impact of more realistic immune landscapes through simulating 40 years of epidemics recurring every 4 years where individuals who gain immunity through infection or vaccination lose their immunity after an average of 4.5 years; an approximation consistent with the estimated duration of protection from oral cholera vaccine (5 years, [6]) and an average lifespan of 50 years
Summary
Reactive vaccination has become an important tool in the fight against diseases such as measles [1], meningitis [2], foot-and-mouth disease [3] and cholera [4]. The performance of different vaccine allocation strategies is driven by a number of complex factors, including heterogeneity in transmission potential across the area, the immune landscape [14,15] and the epidemiologic connectivity (i.e. spatial coupling) between infected areas. We build upon the results of previous work, which mostly focuses on influenza vaccination strategies [15,20,21,22], to show how three intuitive and simple reactive vaccination strategies may perform over different epidemiologic landscapes when vaccine supply is limited. We use simple metapopulation models of a choleralike disease (figure 1) to explore how to allocate vaccine both proactively and reactively when supply is limited in the presence of a transmission hotspot and one or more non-hotspots. We estimate the reduction in epidemic size achieved by three simple strategies over a range of epidemiologic settings: (i) targeting vaccine at the transmission hotspot, (ii) targeting vaccine at non-hotspot(s) and (iii) allocation proportional to population size ( pro-rata)
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