On the scaling patterns of infectious disease incidence in cities

TL;DR

This study examines how infectious disease rates, especially for sexually transmitted diseases, increase with city size, revealing that less transmissible diseases scale more steeply and are more variable across urban areas.

Contribution

It introduces a mathematical model and a new urban scaling theory to explain how disease transmissibility influences scaling patterns in cities.

Findings

Less transmissible diseases scale more steeply with city size

Disease incidence is more variable across similarly sized cities

Theoretical models link transmissibility to scaling behavior

Abstract

Urban areas with larger and more connected populations offer an auspicious environment for contagion processes such as the spread of pathogens. Empirical evidence reveals a systematic increase in the rates of certain sexually transmitted diseases (STDs) with larger urban population size. However, the main drivers of these systemic infection patterns are still not well understood, and rampant urbanization rates worldwide makes it critical to advance our understanding on this front. Using confirmed-cases data for three STDs in US metropolitan areas, we investigate the scaling patterns of infectious disease incidence in urban areas. The most salient features of these patterns are that, on average, the incidence of infectious diseases that transmit with less ease-- either because of a lower inherent transmissibility or due to a less suitable environment for transmission-- scale more steeply with population size, are less predictable across time and more variable across cities of similar size. These features are explained, first, using a simple mathematical model of contagion, and then through the lens of a new theory of urban scaling. These theoretical frameworks help us reveal the links between the factors that determine the transmissibility of infectious diseases and the properties of their scaling patterns across cities.