The R0 assumes a previously unexposed population, where everyone is vulnerable.
Ir's not as simple as 'how many people did Jane give it to?'
It's made up of three factors - how long someone is infectious , how many of the people the infected person passes the disease on to (the secondary attack rate) and the number of people the infected person has been in contact with.
The R0 starts to become less useful as the proportion of the population whomhave previously been exposed starts to rise. When instead of R0, you need to use Re instead:
"The effective reproduction number, Re, sometimes also called Rt, is the number of people in a population who can be infected by an individual at any specific time. It changes as the population becomes increasingly immunized, either by individual immunity following infection or by vaccination, and also as people die.
"Re is affected by the number of people with the infection and the number of susceptibles with whom infected people are in contact. People’s behaviour (e.g. social distancing) can also affect Re.
"The number of susceptibles falls as people die or become immunized by exposure. The sooner people recover or die, the smaller the value of Re will be at any given time.
"Unfortunately, the symbol R0 is often used in publications when Re is meant. This can be confusing"