Vitamin A supplementation (VAS) and vaccines are the most powerful tools to reduce child mortality in low-income countries. However, we may not use these interventions optimally because we disregard that the interventions may have immunomodulatory effects which differ for boys and girls and which may interact with the effects of other interventions. I have proposed the hypothesis that VAS and vaccines interact. This hypothesis is supported by randomised and observational studies showing that the combination of VAS and DTP may be harmful. I have furthermore proposed that VAS has sex-differential effects. VAS seems beneficial for boys but may not carry any benefits for girls. These findings challenge the current understanding that VAS and vaccines have only targeted effects and can be given together without considering interactions. This is of outmost importance for policy makers. The global trend is to combine health interventions for logistic reasons. My research suggests that this may not always be a good idea. Furthermore, the concept of sex-differential response to our common health interventions opens up for a completely new understanding of the immunology of the two sexes and may imply that we need to treat the two sexes differently in order to treat them optimally possibly also in high-income countries. In the present proposal I outline a series of inter-disciplinary epidemiological and immunological studies, which will serve to determine the overall and sex-differential effects of VAS and vaccines, the mechanisms behind these effects, and the basis for the immunological difference between boys and girls. If my hypotheses are true we can use the existing tools in a more optimal way to reduce child mortality without increasing costs. Thus, the results could lead to shifts in policy as well as paradigms.