A study of SARS-CoV-2 evolution in Italy: from early days to secondary effects after social distancing

Marco Claudio Traini; Carla Caponi; Riccardo Ferrari; Giuseppe Vittorio De Socio

The outbreak of the severe acute respiratory syndrome coronaviruses (SARS-CoV-2) [1] [2] [3] has been defined pandemia on March 11, 2020 due to the global spread. The respiratory droplet transmission is the main contagious route of SARS-CoV-2, and it can also be transmitted through contact [4] . The latency period is generally from 3 to 7 days, with a maximum of 14 days [5] and unlike SARS-CoV, SARS-CoV-2 is contagious during the latency period [6, 7] . In Europe, Italy is becoming a particularly alarming and interesting place to study the evolution of the epidemic also thanks to the detailed information offered the Italian Health organizations and the relevant control measures adopted to prevent transmission. Based on Chinese experience and the estimation of transmission models published by Tang et al. [8, 9] , we develop an approach to the evolution of the SARS-CoV-2 during the early stages of transmission in Italy (see also ref. [10] ). Our finding may be useful for inference, forecasting or scenario analysis. Despite the fact that epidemic is changing rapidly and our results have to be considered an estimation, the models we are using can be considered predictive and useful for the interpretation of such an unexpected event. A key factor influencing strongly the SARS CoV-2 outbreak evolution is the (effective) viral reproduction number (R 0 ) defined as the mean number of secondary cases generated by a typical infectious individual on each day in a full susceptible population, the mean R 0 ranges was estimated from 2.24 (95%CI: 1.962.55) to 3.58 (95%CI: 2.894.39) in the early phase of the outbreak in Cina [11] . Our aim is the study of the relevant parameters of control strategies lowering the reproduction rate of SARS-CoV-2 and mitigating the consequences of the restoration of social normalcy. This last aspect is studied in some detail to investigate the measures to be taken to start restoration without running into strong secondary events [12] . 1 . Diagram of the model simulating the novel Coronavirus (Sars-CoV-2) infection in Italy. The population is stratified in Susceptible (S), exposed (E), infectious but not yet symptomatic (pre-symptomatic) (A), infectious with symptoms (I), hospitalized (H) and recovered (R), quarantined susceptible (Sq), isolated exposed (Eq) and isolated infected compartments. Interventions like intensive contact tracing followed by quarantine and isolation are indicated (cfr. ref. [8] ).