Vaccination and Epidemics in Networked Populations - An Introduction

"[T]here is a strong public health and scientific rationale for studying behavior-disease-vaccination interactions."

In the context of recent research exploring the implications of individual behaviour and heterogeneous contact patterns in networked populations, as well as the feedback loops that exist between vaccinating behaviour and disease propagation, this article introduces a journal issue on vaccination and epidemics in networked populations.

It begins by looking at network science, noting, for example, that "the consideration of multilayer or interdependent networks is crucial for a comprehensive treatment of vaccination and epidemics in networked populations." Furthermore, "technological breakthroughs in the acquisition and storage of vast amounts of digitized data have significantly aided the progress in our understanding of both vaccination and epidemic spreading....In particular, the synergies between mathematical modeling and theoretical explorations and data-driven research, coupled with taking into account feedbacks between disease, behavior and vaccination, are likely the future of complex systems research aimed at a better understanding of vaccination and epidemics in modern human societies."

Although "vaccination efforts on networks are more effective if random vaccination is combined with targeted vaccination and the vaccination of acquaintances", vaccination is typically considered to be voluntary, and there are challenges such as vaccine refusal/hesitancy - not a new phenomemon. Human behaviour can be difficult to mathematically model, but models that include behaviour can explain the data more effectively compared to models that neglect behaviour." Figure 3 in the article illustrates the behaviour-disease-vaccination interactions, which form a feedback loop. In brief, the fast diffusion of a disease will increase risk perception, based on which people feel an incentive to take protective measures, i.e., to vaccinate. In turn, the spreading of the disease is suppressed, to the point where risk perception drops again and people opt not to vaccinate. The article reviews a phenomenological model to capture the impact of adaptive human behaviour on disease transmission. ("The model is phenomenological because, although it is motivated by psychological factors, the equations only describe the impact of such psychological factors on the transmission rate without actually modeling specific psychological processes such as individual cognition or social learning.")

Another theoretical framework to account for behaviour-disease-vaccination interactions is evolutionary game theory, which is examined next. In short, individuals who do not vaccinate but still benefit from herd immunity, which is generated by a large fraction of those who do vaccinate, individually make the better choice, i.e., like free-riding in evolutionary games. The article also discusses network epidemiology, which "draws on synergies between large-scale simulations and data-based network construction, the outcome of which is typically an agent-based model that evolves in a spatially structured population. The GLobal Epidemic and Mobility (GLEAM) model...is a particular example that integrates census and mobility data in a fully stochastic metapopulation network model that allows for the detailed simulation of the spread of influenza-like illnesses around the globe." As reported here, the resulting mathematical models can be generalised and adapted to describe diffusion of information, the spreading of rumours and scientific memes, the emergence of online virality, and many other phenomena that are inherent to modern existence, both off and online.

Areas of further research may include "the consideration of mathematical and statistical laws that govern the coevolution of the network and the evolution of vaccination or of the epidemic process that unfolds....Further in terms of the interaction network, recent research has revealed that social networks are almost inherently interdependent and layered, in the sense that a particular network of contacts seldom exists in isolation....It remains subject to future research to reveal to what degree this influences our ability to predict and control the epidemics, and what are the consequences of vaccination abstinence in one social group for others that are codependent."

In conclusion: "vaccination and epidemics in networked populations merit outstanding attention from the research community, not only for their obvious importance to the wellbeing of our societies, but also from the purely scientific point of view due to the many open problems and fascinating challenges that remain to be addressed. The triptych The Garden of Earthly Delights..., painted...around the turn of the 16th century, is a lucid portrayal of events that may transpire if we fail to keep the public health in the best possible order, and if we fail to cooperate in vaccinating our children and working together to minimize the risk of epidemics."