Papers p 1435 Immunisation has led to spectacular reductions in mortality in both developed and developing countries.1 However, we know too little about the overall effect of vaccines. We have taken vaccines and schedules that are effective in developed countries with low levels of childhood mortality and used them in populations with high death rates without studying their effect on total mortality. In this week's BMJ Kristensen and colleagues report some startling results from a prospective cohort study in Guinea-Bissau, which was performed in remarkably difficult circumstances (p 1435).2 Their findings show that both BCG and measles vaccines halved child mortality—a spectacular effect that should be exploited to reduce substantially the number of children dying in developing countries. However, the combination of diphtheria, pertussis, and tetanus (DPT) and polio vaccines seemed to increase mortality (mortality ratio 1.84 (95% confidence interval 1.10 to 3.10)). The increase in mortality from the DPT and polio vaccines is worrying, but these are preliminary findings. For ethical reasons, this was a follow up study and not a controlled trial and, understandably, the response rate was low (66%). In addition, the effect of this immunisation on mortality only just reached statistical significance, with a lower 95% confidence limit of 1.10 (where 1.00 is no …