  Effective strategies of vaccine prioritization are essential to mitigate the
impacts of severe infectious diseases. We investigate the role of infection
fatality ratio (IFR) and social contact matrices on vaccination prioritization
using a compartmental epidemic model fueled by real-world data of different
diseases and countries. Our study confirms that massive and early vaccination
is extremely effective to reduce the disease fatality if the contagion is
mitigated, but the effectiveness is increasingly reduced as vaccination
beginning delays in an uncontrolled epidemiological scenario. The optimal and
least effective prioritization strategies depend non-linearly on
epidemiological variables. Regions of the epidemiological parameter space, in
which prioritizing the most vulnerable population is more effective than the
most contagious individuals, depend strongly on the IFR age profile being, for
example, substantially broader for COVID-19 in comparison with seasonal
influenza. Demographics and social contact matrices deform the phase diagrams
but do not alter their qualitative shapes.
