  The bacterium Group A Streptococcus (Streptococcus pyogenes, GAS) is a
human-specific pathogen and a major cause of global morbidity and mortality.
Despite decades of research our knowledge of GAS infection and immunity is
incomplete, hampering vaccine design and other efforts to reduce disease
prevalence. Epidemiological studies indicate positive associations between the
prevalence of GAS-related disease, the diversity of circulating strains and the
degree of poverty in host populations. However, the infection and immune
mechanisms underlying these associations are not clear. In this work, we use an
agent-based model to demonstrate that observed diversity and prevalence are
best accounted for by the hypothesis that GAS infection confers enduring
strain-specific immunity, with reduced or absent cross-protection against
infection by other strains. Our results suggest that the success of GAS
vaccines will depend on their ability to elicit long-lasting cross-protective
immunity over multiple strain types.

Populations and Evolution