Hypochondriacs might be good for everyone’s health, to judge from a study that’s just been published. The paper describes a computer model of the uptake of vaccinations by a population, and shows that just a few individuals whose fear of the disease prompts them to take a vaccine can trigger others to do the same. As a result the population as a whole is more likely to reach a critical mass of vaccinated people, in other words reach “herd immunity”.
With the appearance of potentially fatal infectious diseases such as swine flu and bird flu (H5N1), not to mention the threat of AIDS and other sexually transmitted diseases, forecasting epidemics has become a central concern in modern medicine. It’s vital both for developing strategies for mitigation and for assessing the emergency measures that will be needed if a major outbreak occurs.
Time was when modelling epidemics tended to involve rather simple models in which virtual people – or other animals – were situated on a checkerboard-like grid and infected their neighbours. While this basic idea – that you risk catching a disease from others with whom you come into close proximity – remains at the core of all epidemic computer models, these days programmes such as the US National Institutes of Health’s Models of Infectious Disease Agent Study (MIDAS) make use of extremely sophisticated schemes that try to capture the way real people move around (international travel makes it easy for diseases to hop continents), how they are connected in social networks, and how they make decisions about prevention or avoidance.
The trick with these methods is to not lose sight of the wood for the trees: to distinguish the kinds of things one might expect in the real world from outcomes that just depend on a specific model’s details. That’s why relatively simple models (albeit complex compared to those a decade or so ago) like the one used by Xiao-Tao Liu of Lanzhou University in China and colleagues are still valuable.
Liu and coworkers say that most epidemiological models tend to assume that people all behave in much the same way, namely that they do what others do. But that may not actually be the case. The researchers propose that a few will be “committed” to vaccination, perhaps because they have a low threshold for feeling at risk (or strongly held convictions), and they will want to be immunised as soon as they hear that someone is infected. If the model of Liu and colleagues stands up, it implies that these committed individuals can have a disproportionate effect, seeding vaccine uptake in others while also disrupting clusters of people forming who reinforce each other’s refusal to receive a vaccine.
There are two important messages to take away from a study like this, regardless of whether one thinks the model captures any real-life behaviour. First, epidemic outbreaks may be very sensitive to the details of how we make our choices about health, and in particular how those choices come about through the influence of other people. Second, the outcomes are often non-intuitive: you’d never guess, by looking at the decision-making rules alone, what the overall outcome might be.
The basic problem for vaccination programmes is that of the free-rider. If there’s an individual cost to vaccination, whether it is inconvenience or a perceived risk of the vaccine itself, people might opt not to get vaccinated themselves but to count on the protection that comes from everyone else doing so. If too many people do this, the herd immunity crumbles and the whole population stands at risk – as seemed to be happening during the scare over the (spurious) link of the MMR vaccine to autism.