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Abstract
Although measles incidence has reached historic lows in many parts of the world, the disease still causes substantial morbidity globally. Even where control programs have succeeded in driving measles locally extinct, unless vaccination coverage is maintained at extremely high levels, susceptible numbers may increase sufficiently to spark large outbreaks. Human mobility will drive potentially infectious contacts and interact with the landscape of susceptibility to determine the pattern of measles outbreaks. These interactions have proved difficult to characterise empirically. We explore the degree to which new sources of data combined with existing public health data can be used to evaluate the landscape of immunity and the role of spatial movement for measles introductions by retrospectively evaluating our ability to predict measles outbreaks in vaccinated populations. Using inferred spatial patterns of accumulation of susceptible individuals and travel data, we predicted the timing of epidemics in each district of Pakistan during a large measles outbreak in 2012-2013 with over 30 000 reported cases. We combined these data with mobility data extracted from over 40 million mobile phone subscribers during the same time frame in the country to quantify the role of connectivity in the spread of measles. We investigate how different approaches could contribute to targeting vaccination efforts to reach districts before outbreaks started. While some prediction was possible, accuracy was low and we discuss key uncertainties linked to existing data streams that impede such inference and detail what data might be necessary to robustly infer timing of epidemics.
Highlights
Despite considerable progress in reducing the global burden of measles infection over the last decades [1], measles remains a leading cause of childhood mortality in many low-income countries [2]
Where measles is locally extinct, yet susceptibility is sufficiently high, generally as a result of the accumulation of unvaccinated births, outbreak risk depends on the probability of contact with an infected individual, shaped by patterns of spatial connectivity resulting from human travel [6]
We focused on data originating from the six most populated provinces in Pakistan (Balochistan, Islamabad, Federally Administered Tribal Area (FATA), KP, Punjab and Sindh) that reported vaccination coverage estimates (Table S2, Fig. S1)
Summary
Despite considerable progress in reducing the global burden of measles infection over the last decades [1], measles remains a leading cause of childhood mortality in many low-income countries [2]. Measles control depends on the delivery of an effective and inexpensive vaccine that confers life-long protection. Challenges associated with vaccine delivery mean that spatial heterogeneity in coverage persists even in countries with otherwise effective vaccination programs allowing sporadic outbreaks to occur [3]. Local susceptibility is defined by the history of vaccination coverage, exposure to natural infection and birth rates [6] and will determine whether measles goes locally extinct [6, 7]. Where measles is locally extinct, yet susceptibility is sufficiently high, generally as a result of the accumulation of unvaccinated births, outbreak risk depends on the probability of contact with an infected individual, shaped by patterns of spatial connectivity resulting from human travel [6]
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