AGI – Concern for the epidemic trend in Italy returns, with the evident growth of the last few weeks. Concern in some way endorsed by the most recent data on the Rt contagion index, which returned, according to the latest report by the Ministry of Health and the Higher Institute of Health, a little above 1 at the national level, to be exact at 1.01. The alert level is triggered just when you exceed 1, because it means that the contagion grows: every positive person potentially infects more than one, so it takes very little for the curve to bend upwards and take on an exponential dimension, the main fear of health authorities around the world.
But what exactly is RT?
This is the calculation on the reproduction index of a disease, elaborated through complex algorithms and evaluated over a suitable period of time, for example on a weekly basis. At the beginning of the pandemic everyone remembers scientists talking instead of “R with zero”, or R0. This is always the reproduction index of a virus, but only in the initial stages, when no specific interventions (pharmacological or otherwise) are normally performed to control the infectious phenomenon.
R0 essentially represents the transmission potential, or transmissibility, of an uncontrolled infectious disease. This R0 value is a function of the probability of transmission by single contact between an infected and a susceptible person, the number of contacts of the infected person and the duration of the infectivity.
The definition of the net reproduction number (Rt) is equivalent to that of R0, with the difference that Rt is calculated over time. RT allows for example to monitor the effectiveness of interventions during an epidemic. R0 and Rt can be calculated on a statistical basis starting from a daily case incidence curve (the number of new cases, day by day).
To calculate R0 or Rt it is not necessary to know the total number of new infections per day. It is not certain, as epidemiologists are struggling to say, the only “thermometer” we have to understand the progress of the infection, indeed: it is no coincidence that the monitoring parameters of the control room set up by the ministry and ISS also take into account more readable data such as those of hospital loads.
For two substantial reasons: the first is that Rt, evaluating a trend over several days with respect to the previously acquired data, risks being overestimated in areas with low incidence, where even a small rise can cause the index value to soar, as a success. in recent months to practically Covid-free regions but victims of some sporadic outbreaks (in particular Umbria and Val d’Aosta), which temporarily found themselves an Rt higher than that of Lombardy.
The second criticality, as explained by the ISS in a series of Faqs published on its portal, is that Rt is calculated only on symptomatic cases. For this only in the last week has the value returned above 1, despite being now over a month that we are witnessing a substantial growth of cases throughout the Peninsula. A calculation criterion that, if it could have been good at the beginning, when essentially only symptomatic or even serious cases were diagnosed, marks the step now that the vast majority of new positives are asymptomatic or at most paucisymptomatic.
The RT calculated only on symptomatic cases, the ISS admits, “while remaining the most reliable indicator at a regional level and comparable over time for monitoring transmissibility, could slightly underestimate the real transmission of the virus at national level”.
The reason for using this criterion lies in the fact that the statistical method of calculating Rt is robust if it is calculated on a number of infections identified according to criteria that are sufficiently stable over time.
Region by region, the criteria with which symptomatic cases are identified or with which the most serious cases are hospitalized are constant, and the number of this type of patients is therefore closely linked to the transmissibility of the virus. On the contrary, the detection of asymptomatic infections depends very much on the ability of the prevention departments to carry out screening and this can vary a lot over time.
For example, the ability to screen can significantly increase as the total incidence of the disease and therefore the workload on the healthcare system decreases. The result is that a greater or lesser increase in asymptomatic cases found does not depend on the transmissibility of the virus but on the number of analyzes performed.
Therefore, it was decided to estimate the transmissibility of SARS-COV-2 in the various Italian regions starting from the curve of daily symptomatic cases as it was less influenced by the change that occurred in Italy in the diagnostic assessment policies on asymptomatic subjects ( which in recent weeks, as mentioned, constitute the majority of diagnosed cases).
Despite these “defects” upstream, however, the Rt index remains a figure considered indicative, paradoxically even more than the mere calculation of the new daily data. It serves, for example, to make a comparison with the first wave: although the number of cases reported daily is numerically similar if not higher than that of the phase at the turn of the lockdown, the epidemiological phase is completely different, and this is demonstrated by the RT which in the the most dramatic weeks of the epidemic even exceeded 2, which led, as indeed happened, to very rapid doubling times.