Several authors have mentioned the importance of allowing for the susceptible proportion of the population. This seems to me especially difficult to estimate, especially at a local, that is, spatially disaggregated, level. Firstly, while we do know local vaccination numbers, accurate estimation of asymptomatic or mildly asymptomatic case-numbers is problematic, even with the availability of data from high-quality randomised prevalence surveys such as the REACT study (Riley et al., 2021). Secondly, susceptibility is not even a binary value.

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