Strain coexistence and competition for pathogens with asymmetric cross-immunity and waning immunity

The article examines how different pathogen strains can coexist, focusing on cross-immunity—the protection that infection with one strain provides against another. Cross-immunity between strains is often asymmetric, meaning protection differs in each direction. Researchers analyzed a mathematical model with two strains that accounts for asymmetric cross-immunity and waning immunity over time. Using this model, they determined conditions under which strains can stably coexist and calculated their prevalence at equilibrium. They found that temporary reduction in transmission can lead to extinction of the weaker strain. Their analysis provides an explanation for the disappearance of the Yamagata lineage of influenza B during the COVID-19 pandemic.