The problem is that R₀ cannot be directly measured, as it does not represent a real event. We therefore have to calculate it through some other (measurable) quantity. This is where the average age at infection comes in.

The average age at infection reflects many of the basic biological and social elements reflected in R₀. It can be intuitively understood that the larger the basic reproductive rate of an infection (R₀) (remember its definition in slide 10) the faster it will spread in society and, therefore, the younger the average age at infection prior to the introduction of vaccination (A).

It has been demonstrated that, under certain assumptions, the basic reproductive rate (R₀) relates in a simple way to the average age at infection prior to the introduction of vaccination (A) and to the life expectancy of the population (L).

It is approximately:

R₀=L/A, (developed countries),

R₀=1+L/A, (developing countries).