  In diseases with long-term immunity, vaccination is known to increase the
average age at infection as a result of the decrease in the pathogen
circulation. This implies that a vaccination campaign can have negative effects
when a disease is more costly (financial or health-related costs) for higher
ages. This work considers an age-structured population transmission model with
imperfect vaccination. We aim to compare the social and individual costs of
vaccination, assuming that disease costs are age-dependent, while the disease's
dynamic is age-independent. A model coupling pathogen deterministic dynamics
for a population consisting of juveniles and adults, assumed to be rational
agents, is introduced. The parameter region for which vaccination has a
positive social impact is fully characterized and the Nash equilibrium of the
vaccination game is obtained. Finally, collective strategies designed to
promote voluntary vaccination, without compromising social welfare, are
discussed.
