Figure 4. Positive consequences of social contagion on public health.

Panel (a) depicts the final epidemic size (t = 120) as a function of the parameter a describing the extent of the role that social contagion, in comparison to the risk of infection, plays in an individual’s vaccination decision. The circle marks the estimated value of a₀ ≈ 0.24 inferred from our real data. Panels (b) and (c) plot the population aggregate levels of vaccination and infection, predicted by our dueling contagion model, with halving (a = a₀/2, the triangle in panel a) versus doubling (a = 2a₀, the square in panel a) the relative effect of peer influence on vaccinating decisions of individuals. It seems rational for one to decide whether or not to be vaccinated according to the level of disease prevalence, but paradoxically the health outcome is worsened, as self-interest and social optimum are at odds in this case. In contrast, herd behavior, induced by social influence, rapidly boosts the uptake level and thus most improves the health outcome: the epidemic size could be reduced by half if the spread of vaccination behavior is driven only by social contagion (a = 1). Simulated results are based on the corse-grained version of the dueling contagion model, eqs (5–8), using the best estimated values of the model parameters.