Past exam of the mathematical and Computational Biology course of the University of Cambridge / 2025 / Paper 2 / 4 / c / Solution

A susceptible person must have the same annual force of infection at every age. Thus the difference between age groups is their length of exposure, not an age-dependent infection hazard. To interpret one fitted constant as a historical annual force of infection, also assume that this common rate was sufficiently stable over the calendar years represented by the sampled birth cohorts. Otherwise an age pattern can reflect past outbreaks or cohort exposure, and the fitted constant is only a model-dependent summary. Representative sampling and comparable antibody detection across ages are additional conditions on interpreting the observed seroprevalence.