An earlier study of vaccination behaviour, in which people were assumed to imitate others who adopted “successful” strategies, predicted that the proportion of a population that takes a vaccine could oscillate over time – first high, then low, and so on. Such oscillations are often a sign of a precarious balance of influences, which push the system first one way and then the other. You find them, for example, in the interdependence of predators and their prey in ecology, or in traffic flow, where they might lead to a jam. In such cases, small influences can have disproportionately big effects. That’s one reason why Liu and colleagues wondered whether individuals committed to vaccination might affect the overall behaviour markedly even if they were few in number.
In the researchers’ model, each individual (called an agent) decides whether to vaccinate or not in successive “rounds” of decision, which can correspond for example to successive “flu seasons”. This choice is made after seeing the outcomes of decisions made both by oneself and one’s immediate neighbours. If, for example, you didn’t vaccinate and did get infected, then in the next round you might copy someone who did get vaccinated.
But there is also a certain fraction of agents dispersed through the population that is always committed to vaccination. The researchers find that even a relatively small proportion of these agents (say, 5-10%) can significantly reduce the scale of an epidemic. What’s more, the effect is much stronger when all the agents are assigned specific locations on a checkerboard array – that is, in each round they “meet” the same the people they interact with regularly, like we do in reality – rather than being randomly reshuffled into a homogeneous mixture. Above a certain threshold proportion of committed agents, an epidemic can be confined to patches, rather than spreading through the whole population. And the committed agents can disrupt the appearance of clusters of free-riders, which might otherwise seed the emergence of a global epidemic.
Do such committed individuals really exist, though? It seems plausible, but Liu’s colleague Zhi-Xi Wu at Lanzhou admits it is still a supposition. However, if they do then the results imply that they can act as influential “role models” for others, and are most effective if they are dispersed quite uniformly in the population. The same should apply to other behaviours: a few committed individuals who always practice safe sex might limit the spread of sexually transmitted diseases, and perhaps even non-medically-related behaviours such as responsible energy use and recycling could be propagated this way – not just among individuals but, say, businesses too. That could be an empowering message: one all too often hears the lament that individuals can’t make much difference to the world, but the power of social feedback implies that you might have more influence than you think.