Why have numbers of reported chlamydia cases been going up for at least a decade in many developed countries? In this issue, Rekart and Brunham (see page 87) and Miller (see page 82) debate whether or not the observed trend means that public health measures to control chlamydia are failing.1 2 Their opinions are “no” and “we don’t know”.

Rekart and Brunham argue that “arrested immunity” is the main explanation for the increasing trend.1 The starting point for this debate is their hypothesis that widespread early treatment has impaired the development of immune responses that would protect against reinfection, resulting in a paradoxical increase in population susceptibility to chlamydia.3 At a population level, they argue that this phenomenon would fit observed trends in reported cases from British Columbia, Canada and some European countries, where declines in the late 1980s and early 1990s have reversed and increased continuously since. Miller argues, however, that trends …