The theory underlying the 2020 pandemic computer models, such as the “Imperial Model” can be traced back to the Kermack and McKendrick paper “A contribution to the mathematical theory of epidemics” which was published August 1, 1927. Stemming from the earlier “germ theory” in the second half of the nineteenth century. [1]
The paper considers there are “various factors” that govern spread of disease from “affected to the unaffected”, and acknowledged the “stage of the sickness” affects transmissibility, and that once one recovers from infection “the number of unaffected members of the community becomes reduced” thus reducing the pool of “susceptible individuals”.
The paper works from the assumption that ” all members of the community are initially equally susceptible to the disease, and it will be further assumed that complete immunity is conferred by a single infection.”
Thus begins the Susceptible, Infectious, Recovered (SIR) modelling framework which “partitions a population into at least three groups: those who are susceptible to future infection (S), those who are currently infectious (I), and those who have been removed from the infectious group through recovery or death (R). [1]