Mathematical Models. Mathematics can be used to "model", or represent, how the real world works. Mathematical modelling can be used for a number of different reasons. It is used for tactical decisions by managers and for strategic decisions by planners. Deterministic models ignore random variation in vaccinated and unvaccinated population. Level of understanding in population about post vaccination output and success rates.
A text book explaining modelling approaches and with additional references: Keeling and Rohani, Modelling Infectious Diseases in humans and animals, Princeton Univ Press, Chapter 8.