Vaccine interference may be intra- or inter-vaccine in nature. Intra-vaccine interference is determined by the nature and dose of the individual vaccine valences, the nature and quality of any additives and the pharmaceutical formulation of the product. Additionally, vaccinee factors including the presence of pre-existing immunity, the stage of immunological maturation, genetic and environmental background may also determine interference. The vaccine schedule and mode of delivery are further contributory factors. In practice, the phenomenon of vaccine interference argues that individual vaccines should not be combined or associated in the absence of specific data sheet recommendations to do so.