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

Our results on the numerical values of the (effective daily) reproduction number R 0 show that a social distancing of 2 contacts per day is not effective to reduce its value below 1.7, even with a test rapidity t I = 1 day to quarantine infected people. Our study suggests that the two parameters have to be activated at the same time. Requesting strict social distancing without reaching a critical value for R 0 in a reasonable number of days risks a failure and the lose of faith in the adopted mea- sures. The most favorable Scenarios (D) and (E) (cfr. lower panel of Fig. 4 ) need from 10 to 15 days before reaching values of R 0 < 1 despite a screening rapidity of 8 hours (from a starting point of 3 days). The inertia of the distribution can force the time interval up to one month before seeing visible effects. The improvement of rapid diagnosis of SARS-CoV-2 with automation and large number of sample processing, is essential in order to implement infection prevention measures lowering R 0 significantly. After March 8th, the measures proposed to reduce the social distancing in Italy have been enhanced each week in order to lower the contact rate (already as low as 2 contact per day, as assumed in the scenarios of Section III. The combined effect of the imposed isolation and a strong reduction of screening time can be seen in the lower curve of Fig. 5 (scenario (D) ). The cumulative effect reduces the delay from 10 days to 5 days to reach R 0 ≈ 1 and in a further 5 days R 0 ≈ 0.15, a value drastically low to see, day by day a 30% reduction of new infected people each day. The corresponding parameters are c ≈ 1 contact per day and t I ≈ 10 hours.