Theory versus Data: How to Calculate R0?

Abstract

To predict the potential severity of outbreaks of infectious diseases such as SARS, HIV, TB and smallpox, a summary parameter, the basic reproduction number R₀, is generally calculated from a population-level model. R₀ specifies the average number of secondary infections caused by one infected individual during his/her entire infectious period at the start of an outbreak. R₀ is used to assess the severity of the outbreak, as well as the strength of the medical and/or behavioral interventions necessary for control. Conventionally, it is assumed that if R₀>1 the outbreak generates an epidemic, and if R₀0). Surprisingly, we find that this value of R₀ calculated from the ILM is very different from the epidemic threshold calculated from the population-level model. This occurs because many different individual-level processes can generate the same incidence and prevalence patterns. We show that obtaining R₀ from empirical contact tracing data collected by epidemiologists and using this R₀ as a threshold parameter for a population-level model could produce extremely misleading estimates of the infectiousness of the pathogen, the severity of an outbreak, and the strength of the medical and/or behavioral interventions necessary for control.