English: Diagram demonstrating spread of covid-19 infection with a basic reproduction number R0 = 4 (see Table 1 below). A- Spread of covid-19 over 3 generations after introduction into a fully susceptible population (1 case can lead to 4 cases and then to 16 cases and so on). B-Expected transmissions of covid-19 if (R0 - 1)/R0 = 1 - 1/R0 = 3/4 of the population is immune. Under such circumstance (achieved through vaccination or natural immunity), all except 1 of the contacts for each case is immune, hence in this circumstances one case will leads to only 1 successful transmission of the infection. This will lead to constant incidence over time. If a greater proportion of the population is immune to covid-19, then the incidence will decrease. On this calculation and basis, (R0 - 1)/R0 is known as the herd immunity threshold.