R_o = b x k x D where b is the attack rate, k is the number of potentially infectious contacts the average person in the population has per unit time and D is the duration of infectivity of an infected person.

An approximation of R_o can be calculated as 1 + the average lifespan of the individual’s in the population/the average age at infection.

If R_o < 1, then every new generation of infection will affect fewer individuals and eventually the disease will die out. The value of R_o and the % of the population that is vaccinated affects disease spread and die out.

If R_o = 1 then approximately the same number of individuals are infected with every new generation causing endemicity.