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Vaccination is unusual among medical interventions in that its benefits extend well beyond the person treated. When enough of a population is immune, transmission chains break and even the unvaccinated are protected, a phenomenon economists recognise as a positive externality. Because individuals weigh only their private costs and benefits, the socially optimal level of vaccination is higher than the level a free market will deliver. This is the standard justification for public subsidy, school entry requirements and, in some jurisdictions, mandates. The argument has a mirror image, however. As a disease becomes rare, the perceived private benefit of vaccination falls while the perceived risk of side effects remains constant, so uptake can decline precisely when a programme is succeeding. Public health authorities therefore face a durable paradox: the more effective their campaigns, the harder they must work to sustain them.