Covid-19: what can the Rt number tell us and why is it so important?

The first of which is precisely the importance of calculating the Rt number: “It is the only one of the 21 indicators capable of helping us understand the epidemic’s trend.  We often start arguing about numbers, but we forget what their real meaning is which, in our case, is being able to tell what will happen in the next two weeks. It is clearly a system that brings with it uncertainties – said our researcher – but it is the best tool available to us, solidly built over the years and widely recognized by the scientific community”. During the press conference introduced by the President of the Higher Health Council, Silvio Brusaferro, Merler also showed for the first time a graph with the estimates made for the period from October 13 to early November. Prediction band estimates are shown in red, while the dotted line shows how intensive care bed occupancy has progressed, with both trends largely coinciding. A graph that highlights the validity of the Rt number compared to what is the most important issue produced by Covid-19, namely pressure on the health system.

In the past few days, some media argued that a lower Rt number was due to the inability of the monitoring system to track asymptomatic individuals. But, as we can see from the following graph which shows data from June to November, the Rt number trend for hospitalized (blue line) and symptomatics (red line) is almost the same, with a difference between the two lines due to the greater numbers of young people among those infected with symptoms.

The Director General of Health Prevention of the Ministry of Health, Gianni Rezza, closed Stefano Merler’s speech by highlighting how these numbers have really hit the target and have so far been essential to control the spread of the epidemic. At the end of the conference, it was announced that hose who want to learn more about the indicators and their effectiveness, can visit the Higher Institute of Health website and view a focus entitled “ISS, il sistema di valutazione del rischio, ecco come e perché funziona” (“ISS, the risk assessment system, how and why it works”)