Impact of population behavioural responses on the critical community size of infectious diseases

The critical community size (CCS) is the minimum closed population size in which a pathogen can persist indefinitely. Below this threshold, stochasticity eventually causes pathogen extinction. Here, we introduce a mechanism of behaviour-mediated persistence, by which the population response to the pathogen alters the CCS. We exemplify this with infection transmission and non-pharmaceutical interventions (NPIs) in a population where both individuals and government authorities restrict transmission more strongly when case numbers are higher. This results in a coupled social-ecological feedback between disease dynamics and population behaviour. In a parameter regime corresponding to a moderate population response, this feedback allows the pathogen to avoid extinction in epidemic troughs. The result is a very low CCS that allows long-term pathogen persistence. Hence, an incomplete population response represents a “sour spot” that not only ensures relatively high case incidence but also promotes long-term persistence of the pathogen by reducing the CCS. We illustrate this mechanism for parameters corresponding to severe coronavirus 2 (SARS-CoV-2). Given the worldwide prevalence of small, isolated populations, these results emphasize the need for incorporating behavioural feedback into CCS estimates. Regional elimination and global eradication programs for vaccine-preventable infections could also account for this effect.