We often talk about outbreaks in terms of the proportion of a population that is affected. Important factors in determining the proportion of a population that is affected include population density (the proximity of an infectious person to others influences disease transmission), public health interventions (which reduce the likelihood of exposure), and medical infrastructure/capacity (which determines our ability to treat and prevent death in those that are severely ill).

Population size does not directly impact the proportion of a population that will be affected. Factors that would determine proportion affected, especially when it comes to COVID-19, are the population's demographics (e.g. age and existing chronic medical conditions like heart disease, diabetes, lung disease, immune compromised).

References:

https://www.cdc.gov/coronavirus/2019-ncov/specific-groups/high-risk-complications.html https://experience.arcgis.com/experience/685d0ace521648f8a5beeeee1b9125cd www.census.gov/popclock/world/it https://www.census.gov/popclock/https://www.washingtonpost.com/graphics/2020/world/mapping-spread-new-coronavirus/

Drafted 20 March 2020