Past exam of the mathematics course of the University of Cambridge / 2026 / iii / Paper 207 / 2 / e / i / Solution

The $S\to E$ transition must be redefined. A constant $q_{SE}$ gives each susceptible the same external infection pressure even when no infectious person remains. For influenza, the force of infection should depend on the current infectious prevalence. The progression $E\to I$ and recovery $I\to R$ can retain rates $q_{EI}$ and $q_{IR}$.