This framework can be applied to a metapopulation of arbitrary size and complexity, with the number of equations being linearly related to the number of populations. The dynamics of each population are governed by a set of three equations per pathogen strain, and these equations are interlinked within populations by partial, cross-reactive immunity, and between populations through a movement network. The total number of differential equations for any given system will be three times the number of strains multiplied by the number of populations in the metapopulation.