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As people's response to an epidemic depends on risk perception and behavioral changes affect infection dynamics, timely information, harsh memories of past epidemics and evident symptoms can curb disease diffusion, according to a game theory model awarded with the 2015 Bellman Prize

While epidemiologists' models consider individual behavior during an epidemic as static (they just divide people into responsive, who change their habits because of the infection, and not responsive), in fact people's behavior changes during the course of the epidemic as a function of the perceived risk. Newspaper headlines or personal contacts with infected people may induce individuals to increase hand washing, avoid crowded places or cut travels, thus affecting spreading patterns. Also infected people's behavior can have some effect on infection dynamics, as infected individuals end up forcedly changing their behavior, e.g. staying home instead of going to school or to the office.

An article awarded with the 2015 Bellman Prize (which goes to the best paper published in Mathematical Biosciences over the preceding two years) takes into account these factors in drawing a model based on an evolutionary game theory framework. Piero Poletti (postdoctoral researcher at Bocconi's Dondena Centre for Research on Social Dynamics and Public Policy), Marco Ajelli and Stefano Merler (both Fondazione Bruno Kessler) model people's response to the epidemic as a strategic game, in which the adoption of different strategies is assumed to be driven by the perceived convenience of different behaviors, explicitly dependent on the epidemic dynamics (Risk Perception and Effectiveness of Uncoordinated Behavioral Responses in an Emerging Epidemic, in Mathematical Biosciences, 238, 80-89, doi: 10.1016/j.mbs.2012.04.003).

Infection transmission dynamics interact with people's imitation dynamics in determining the spread of the epidemic, affecting the final epidemic size, the daily peak prevalence and the peak day. When the epidemic is so widespread that responsive people's behavior shows a clearly better payoff, non-responsive people tend to imitate them and become responsive. According to the model, even a small reduction in the number of people an individual is in contact with can make a large difference in the spread of the disease and three factors can determine such a reduction. First of all, timely information is key: when there is a delay between the emergence of an epidemic and people awareness, human response becomes less effective. Memories of past epidemics, furthermore, can make people more responsive. Finally, very evident symptoms, like coughing and sneezing, accelerate behavioral changes.

If the spread of panic can prevent the spread of an epidemic, the contrary is also true and the lack of fear can turn out to be a problem, as in the case of children's vaccinations: after a long period without measles outbursts, for instance, families tend to underestimate the severity of the disease and overestimate the side effects of the vaccination, with the consequence that take up rates decline and the risk of new epidemics becomes real.

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