Fig. 1: Transmission bottleneck estimates under a Wells-Riley model of exposure.

In this model, the level of exposure describes the rate parameter of a Poisson model, such that a person receiving an exposure of 1 would expect to be infected by one virus. a The probability of an individual being infected given the level of exposure. b The proportion of cases of infection in which a single virus initiates infection. c The proportion of cases of infection in which ten or fewer viruses initiate infection. The vertical grey bar provides an estimate of circumstances at the Skagit Choir superspreading event, characterised by the probability of infection. Even under these circumstances, the model suggests that transmission bottlenecks are likely to be small.