'Clustering of exemptions' as a collective action threat to herd immunity.

In this paper, we examine the phenomenon of 'clustering of exemptions' to childhood vaccination, and the dangers this poses both to those exempted as well as the general population. We examine how clusters of exemptions might form through collective action as described by Thomas Schelling, and how religious groups who live in close proximity to one another can "self-select" in a way that exacerbates this phenomenon. Given the growing number of exemptions and the increasing visibility of the anti-vaccine movement, policy makers must be vigilant for dangerous clustering in order to avoid loss of herd immunity. Copyright 2002 Published by Elsevier Science Ltd.