Introduction: Ageing has often been associated with a marked reduction in numbers of bifidobacteria and an increase in numbers of clostridia. In addition, ageing is associated with alterations in immunity and poor response to vaccination. Pre- and probiotics are suggested to restore the gut micro flora ecosystem and modulate immune function. The current study evaluated the influence of a novel probiotic, Bifidobacterium longum by. infantis CCUG 52486, with a potential prebiotic (glucooligosaccharide; GIOS) on the gut microbiota composition and on innate immunity in young (18-35y) and older (60-85y) subjects undergoing influenza vaccination.