Smallpox was probably the single most lethal disease in eighteenth-century Britain but was reduced to a minor cause of death by the mid-nineteenth century due to vaccination programmes post-1798. While the success of vaccination is unquestionable, it remains disputed to what extent the prophylactic precursor of vaccination, inoculation, reduced smallpox mortality in the eighteenth century. Smallpox was most lethal in urban populations, but most researchers have judged inoculation to have been unpopular in large towns. Recently, however, Razzell argued that inoculation significantly reduced smallpox mortality of adults and older children in London in the last third of the eighteenth century. This article uses demographic evidence from London and Manchester to confirm previous findings of a sudden fall in adult smallpox mortality and a rise in the importance of smallpox in early childhood c. 1770. The nature of these changes is consistent with an increase in smallpox transmission in London and Manchester after 1770 and indicates that smallpox inoculation was insufficient to reduce smallpox mortality in large towns. It remains unclear whether inoculation could have operated to enhance smallpox transmission or whether changes in the properties of the smallpox virus drove the intensification of smallpox mortality among young children post-1770.