“…The model can be initialized at any time t 0 prior to the start of the vaccination campaign (December 27, 2020 in Italy) by using the data (for those compartments for which data are available, namely I , H , T , E ), null initial values for V 1 and V 2 , while, as proposed in ( Parolini et al, 2021b ), the Undetected and Recovered compartments are initialized as where IFR( t ) is the Infection Fatality Ratio , CFR( t ) is the time-dependent Case Fatality Ratio and d = 13 days denotes the confirmation-to-death delay. In ( Parolini et al, 2021b ), we considered a constant IFR computed as the weighted average of the age-specific IFR estimates weighted by the population age structure, under the assumption of equal attack rates across age-groups, as proposed in ( Brazeau et al, 2020 ). A better estimate of the reference IFR( t ) at a specific time can be computed by considering the variable percentage of each age-group among the total infected over time (data available on ( Dati della Sorveglianza integrata COVID-19 in Italia, ) since December 8, 2020), namely: where IFR i denotes the infection fatality ratio for age-group i and q i ( t ) is the percentage of infected at time t belonging to age-group i .…”