The effect of the definition of 'pandemic' on quantitative assessments of infectious disease outbreak risk

Singer, B. J.; Thompson, R. N.; Bonsall, M. B.

In the early stages of an outbreak, the term 'pandemic' can be used to communicate about infectious disease risk, particularly by those who wish to encourage a large-scale public health response. However, the term lacks a widely accepted quantitative definition. We show that, under alternative quantitative definitions of 'pandemic', an epidemiological metapopulation model produces different estimates of the probability of a pandemic. Critically, we show that using different definitions alters the projected effects on the pandemic risk of key parameters such as inter-regional travel rates, degree of pre-existing immunity, and heterogeneity of transmission rates between regions. Our analysis provides a foundation for understanding the scientific importance of precise language when discussing pandemic risk, illustrating how alternate definitions affect the conclusions of modelling studies. This serves to highlight that those working on pandemic preparedness must remain alert to the variability in the use of the term 'pandemic', and provide specific quantitative of analysis that we show to be sensitive to the pandemic definition.