Non-specific effects of vaccination are sex-specific. Girls benefit most from beneficial non-specific effects, and suffer most from apparently harmful non-specific effects of vaccination. Non-specific effects of measles vaccination may also show interaction with season, the beneficial effect of measles vaccination being largest in the dry season (unpublished), suggesting that some unknown immunological factors play a role. Season has a large influence on health in West Africa, and both morbidity and mortality, as well as immunologic parameters such as delayed-type hypersensitivity, T-cells, thymus size and measles antibody levels which are associated with season.