I am reading the article Application of Whittle’s stochastic threshold theorem to a chickenpox outbreak and I can't understand the meaning of the term "intensity" in the Whittle threshold theorem.
In addition, they deduce a formula from Whittle's theorem for vaccination: $$p=1-\frac{1}{(1-\epsilon)^{1/a}R_0},$$ but I can't understand where it comes from and what the tolerance $\epsilon$ means.
I really appreciate your help.
