Study on voluntary vaccination reveals interesting complexities

When word of this study gets around, you may start to hear that voluntary vaccination "works." This would not be an accurate statement. There is a new study just now out in PLoS Computational Biology that reveals that under certain conditions, which may actually be quite rare, voluntary vaccinations might lead to the eradication of a disease (contrary to 'popular wisdom'). However, you must realize that the study has some important limitations and the results do not suggest that most (if any) current vaccination issues be voluntary rather than mandatory.

The idea that voluntary vaccination would be ineffective is simple: As vaccination spreads as a practice, the disease addressed by the vaccination becomes less of a threat, and an increasing number of people eventually free-ride on those getting vaccinated. So, at some point, there is a pool of unvaccinated individuals who serve the disease (as a reservoir) rather than their fellow humans.

We can see this effect in both empirical studies and in simulation studies. However, according to the authors of this new paper, the simulation studies have been less than adequate because they make assumptions about the population (and the disease) that do not represent all cases.

Previous studies have suggested that voluntary programs cannot be 100% effective due to the self-interested behavior of individuals. However, most mathematical models used in these studies assume that populations mix homogenously - in effect, that an individual is just as likely to be infected by a complete stranger as by a close friend or family member. But that is not how infections spread with diseases like smallpox or SARS, which are predominantly to close social contacts.

In this new study, Bauch and Perisic analyze "free-rider" effects under voluntary vaccination for vaccine-preventable diseases where disease transmission occurs in a social network. Individuals choose whether to vaccinate based on the risk of infection from their neighbors and any risks associated with the vaccine itself. Neighbors of an infected person will vaccinate as soon as their neighbor's symptoms appear, so when neighborhood size is small, voluntary vaccination results in rapid containment of an outbreak. As neighborhood size increases, a threshold is reached beyond which the infection can break through due to the decisions of neighbours who choose not to vaccinate.

"This approach injects greater realism into the transmission modeling of close contact infections and gives us a much more nuanced picture of how people's behavior influences the effectiveness of voluntary versus mandatory vaccination policies," said Bauch. "For those pathogens that are difficult to transmit, the conventional wisdom that free-rider effects will make eradication difficult under voluntary vaccination may be wrong."

So, in effect, the self interest factor is still at play but in a different way. You see your child die of the disease so you go ahead and get your other child vaccinated, for instance.

The article can be downloaded here.

Perisic A, Bauch CT (2009). Social Contact Networks and Disease Eradicability under Voluntary Vaccination PLoS Comput Biol , 5 (2) DOI: 10.1371/journal.pcbi.1000280