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Only who has the symptoms or also the asymptomatic ones? There is no single way, nor a better system than the others, to calculate the contagion rate Rt, which indicates the number of people who can be infected by those who have an infection. This is exactly what happens also in the case of the new coronavirus, with the value indicated by the Istituto Superiore di Sanità (Iss) different from that calculated by the CovidStat group of the National Institute of Nuclear Physics (Infn): 1.18 the first and almost 3 the second.
“Both calculations are valid and the difference in the results is due to the fact that the algorithms used are different”, observes the physicist Francesco Luchetta, one of the curators of the Facebook page “Coronavirus-Scientific data and analysis”. The ISS data, he continues, “tell us how much individuals with symptoms vary and indicate that they do not vary significantly, probably because the infection is moving into younger age groups. It is very clean data, probably as clean as possible, but it only affects the symptomatic. ” The Infn data, Luchetta continues, include symptomatic, asymptomatic and cured patients and “instead tell us that the total number of cases is increasing, but it is a less clear figure.” Both are calculated correctly and different aspects stand out: “the important thing – he points out – is to know what the differences are”.
The same Iss notes on its website that asymptomatic patients are not included in the calculation of the Rt index since their number “does not depend on the transmissibility of the virus, but on the number of analyzes performed.” That is, it depends on the number of swabs performed and, until a few weeks ago, swabs were performed on those with symptoms consistent with Sars-Cov-2 infection. Thanks to this choice, the calculation of the ISS becomes “cleaner, but with the awareness of having an underestimation”, observes Luchetti.
“There is no more correct formula than another” also for the physicist Giorgio Sestili, founder and one of the curators of the Facebook page that analyzes data from the Covid pandemic since March. “Infn’s calculation uses a more classic formula, considering all cases and cured, but it generates an overestimation because the cured data is very dirty, with percentages that vary from region to region; calculating only the symptomatic ISS eliminates the risk of overestimation, but it is underestimated “.
Wanting to use one algorithm instead of the other is a matter of choice and there are those who, for the sake of clarity, have adopted both calculation methods, as did the German institute of health Robert Koch, who in his epidemiological report reports weekly on both values. “We celebrate – concludes Sestili- that more organizations and research institutes promote analyzes that propose different points of view. The important thing is to be aware that each one presents their results with precision ”.
