As above, we denote strains as subscripts and, in the equation for wi, we sum over all strains j which share at least one allele with the focal strain i. β, σ, and μ are the infection, recovery, and death rates, respectively. γ (as mentioned above) is an indicator of the level of cross-reactive immunity gained by prior exposure to alleles in the target strain. Note that while we depict only one value per demographic parameter (i.e. all strains are functionally equivalent) for clarity of notation, these values could also be written to vary by strain (e.g. βi).