Figure 4.
Trend in the cumulative number of infections, hospitalizations, accesses in intensive care units and deaths observed and expected (i.e., that would have occurred in the absence of the vaccination campaign) during the first eleven months from starting the campaign (please see text, Question 4). Footnote. The analysis was based on the cohort of 9,140,390 citizens from Lombardy, beneficiaries of the RHS, who, having already turned ≥12 years old on 27 December 2020, or celebrating their 12th birthday by 11th November 2021, were to be considered potential vaccine recipients for the current study. Estimates were based on the so-called prevented fraction (PFd = Pd(1-HR)) where Pd is the cumulative proportion of citizens reached by the vaccine up to day (d) (see answer to the question 2) and HR is the risk ratio measuring the association between exposure status and a given outcome (see answer to the question 3). The HR is thought to be invariable over time (although this assumption may be questioned; see the answer to the question 6). In contrast, Pd increased during the campaign. The number of outcomes avoided from campaign starting until the day (d) was calculated by applying to the number of outcomes occurred up to day (d) the PF value calculated up to 14 days earlier. The number of outcomes that would have been observed if the vaccine campaign had not been implemented derived from the outcomes avoided and those observed.