The Figures of Coronavirus Aggressiveness in Lombardy

A STUDY BY THE REGION, COAUTHORED BY ALESSIA MELEGARO, LAYS THE FOUNDATIONS FOR AN ACCURATE MODELING OF THE DIFFUSION OF COVID19

A study by the Lombardy Region, co-authored by Professor Alessia Melegaro, (Bocconi's Dondena Center), provided the first description of the COVID-19 outbreak in Lombardy and made available the basic epidemiological parameters of the virus. Thanks to these parameters, mathematical models were developed to study the dynamics of spread of the COVID-19 infection and disease and to inform containment strategies.
 
In The early phase of the COVID-19 outbreak in Lombardy, Italy (https://arxiv.org/abs/2003.09320), Prof, Melegaro and co-authors analyze the spread of the infection up to March 8, considering the first 5,830 confirmed cases.
 
The authors point out that in 47% of the symptomatic cases hospitalization was necessary and that intensive care was necessary for 18% of those hospitalized. The infection seems to be more frequent in the male population (62% of positive cases) and the median age of these first cases was 69 years.
 
The epidemic curve reported in the work shows that the arrival of the virus in Lombardy dates back to the weeks preceding the detection of the first case, which occurred on 20 February 2020 in Codogno.
 
«Data show the strong aggressiveness of the infection, with a number of secondary cases generated by a single positive case (Basic Reproductive Number: R0) that has reached the level of 4 for the Lombardy region, well above the threshold value of 1, below which the epidemic would disappear.  This means that, before the measures were taken, an infected person was able to infect between three and four people». The number of secondary cases began to plummet with the increase in citizens' awareness and in correspondence with the more restrictive measures, so much so that the most significant and rapid decline was recorded in the red zone of Codogno where social interactions had already slowed down drastically since 21 February. Similar values are also reported for the clusters of Bergamo and Cremona.
 
In the first two weeks after the introduction of the virus, cases were doubling every 3.1-3.5 days, depending on the area, with a Reproductive Number from 2.3 to 3.1. This increased in the following two weeks, before starting to decline in the last ten days of February, although remaining above 1.
 
Studying the transmission chains in detail, the study also established that the average number of days between the onset of symptoms in a positive case and a close contact (serial interval) is 6.6 days and only in 5% of cases exceeds 16 days (note: the parameter does not correspond to the incubation period, which is shorter).
 
Finally, the analysis highlights an important aspect of the viral load of positives, namely that this load is similar in symptomatic and asymptomatic cases, thus suggesting a similar contagion potential.
 
Data collection is still running and the timing of publication of the results is clearly conditioned by the inevitable delay between the onset of symptoms, the obtainment of test results and the recording of the case in the database (7.3 and 3.6 days respectively). As the epidemic progresses, these intervals are reducing, giving a positive signal for the containment of the epidemic in Lombardy. The study was made possible by the self-sacrifice of those who collected data in the field in emergency and dangerous conditions, investigating the time of onset of symptoms, conducting epidemiological investigations and tracing the existing connections in the chain of infection. The authors therefore thank the operators of the hospitals in Lombardy and the members of the Coronavirus Task Force of the Lombardy Region.
 

by Fabio Todesco