Superspreading and the effect of individual variation on disease emergence

There is considerable variability in infectiousness between individuals in a population. This heterogeneity in disease transmission within a population may arise from a number of factors, including differences between individual hosts (e.g. behavior), pathogens (e.g. physiology), environmental characteristics, and their interactions. Because of these many interactions, variation in infectiousness in continuous; this continuous variation is largely ignored in epidemic models, which divide populations into homogeneous subgroups.

In this study, Lloyd-Smith et al. (2005) incorporate heterogeneity in transmission into models of superspreading events. To do so, they introduce the “individual reproductive number” (v), which is the number of secondary infections caused by a given individual. The mean v for a population is R₀ and the variability in v is controlled by an additional parameter (k). Using this framework, variability be very high (k<1) or nonexistent (­k approaches infinity, all ­v=R₀) (see Figure 2a from the paper below). Applying this model to a number of infectious diseases shows that there is always variability in transmission in cases where superspreading events occurred. Further exploring this model reveals that high individual variation in transmission (k<1) favors extinction of a disease because no superspreading events occur (the proportion of superspreaders in the population is low and most individuals transmit disease poorly).

The authors also use their model to assess different potential control measures for diseases. They compare individual-specific measures, where the transmission by a subset of the population is brought to zero, and population-wide measures, where the infectiousness of every individual is decreased by a given proportion. While both measure have the same effect on mean population R₀, the authors conclude that the individual-specific measures are more effective in preventing epidemics because they increase heterogeneity in transmission (which they have shown promotes disease extinction). Though this model considers only heterogeneity in transmission – and the authors acknowledge that heterogeneity also exists in other parts of the process, for instance in susceptibility – it is able to provide both an understanding of the dynamics of superspreading events and recommendations for management.

Lloyd-Smith, J. O., Schreiber, S. J., Kopp, P. E., & Getz, W. M. (2005). Superspreading and the effect of individual variation on disease emergence. Nature, 438(7066), 355-359.