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  Let’s look at the same matter in a more practical way, going back to the imaginary example of slides 5-7.

Here we have: R₀ = 3, R = 1, (S/N)eq = 1/3, and (I/N)eq = P_C = 2/3.

In words: this microorganism in this population would be transmitted, on average, to three susceptibles per each infected case should all members of the population be susceptible (R₀=3); on average, the infection is actually transmitted, to only one susceptible person per infected case (R=1); in this state of equilibrium, one out of three people are susceptible ((S/N)eq = 1/3) and two out of three are immune ((I/N)eq = 2/3); if we could achieve a vaccination level higher than 2/3 we would be able to eventually eradicate the microorganism (P_C = 1 - (1/R₀) = 1 - (1/3) = 2/3 = 67%).