The changing epidemiology of COVID-19: a retrospective modeling study on two years of pandemic in Italy

Abstract

Background. The difficulty in identifying SARS-CoV-2 infections has been a major obstacle to control the COVID-19 pandemic, but also to quantify changes in the proportion of infections resulting in hospitalization, intensive care unit (ICU) admission or death.
Methods. We developed a mathematical model of SARS-CoV-2 transmission and vaccination informed by epidemiological surveillance data to estimate the daily number of infections occurred in Italy between February 2020 and February 2022. Model outcomes are used to assess changes in the SARS-CoV-2 infection ascertainment ratio (IAR), infection hospitalization ratio (IHR), infection ICU ratio (IIR), and infection fatality ratio (IFR), in five different sub-periods associated with the dominance of the ancestral lineages, and Alpha, Delta, and Omicron BA.1 variants.
Results. We estimate that, over the first two years of pandemic, the IAR ranged between 15 and 40% (range of 95%CI: 11-61%), with a peak value in the second half of 2020. The IHR, IIR and IFR consistently decreased throughout the pandemic with 22 to 44-fold reductions between the initial phase and the Omicron period. At the end of the study period, we estimate a IHR of 0.24% (95%CI: 0.17-0.36), a IIR of 0.015% (95%CI: 0.011-0.023) and a IFR of 0.05% (95%CI: 0.04-0.08).
Conclusions. Since 2021, changes in the dominant SARS-CoV-2 variant, the rollout of vaccination, and the shift of infection to younger ages have reduced SARS-CoV-2 infection ascertainment. The same factors, combined with the improvement of patient management and care, contributed to a massive reduction in the severity and fatality of COVID-19.

Joint work with Giorgio Guzzetta, Francesco Menegale, Chiara Sacco, Daniele Petrone, Alberto Mateo Urdiales, Martina Del Manso, Antonino Bella, Massimo Fabiani, Maria Fenicia Vescio, Flavia Riccardo, Piero Poletti, Mattia Manica, Agnese Zardini, Valeria d’Andrea, Filippo Trentini, Paola Stefanelli, Giovanni Rezza, Anna Teresa Palamara, Silvio Brusaferro, Marco Ajelli, Patrizio Pezzotti, and Stefano Merler.