At Least 500 COVID Cases in Lombardy Before Patient 1

ALESSIA MELEGARO, RAFFAELLA PICCARRETA AND FILIPPO TRENTINI AMONG THE AUTHORS OF A STUDY, IN COLLABORATION WITH FONDAZIONE BRUNO KESSLER, THAT SHEDS LIGHT ON THE RAPID SPREAD OF THE VIRUS BETWEEN FEBRUARY AND MARCH 2020

At the time of the identification of Patient 1 in Codogno on February 20, 2020, the SARS-CoV-2 virus was already widespread in the Lombardy region, with sustained transmission in all provinces of Lombardy, characterized by an estimated reproduction number (R0) higher than 2 in all provinces.
 
This is stated in the study “The early phase of the COVID-19 epidemic in Lombardy, Italy” published in the journal Epidemics by researchers from Fondazione Bruno Kessler and Bocconi University (Alessia Melegaro, Director of COVID Crisis Lab, Raffaella Piccarreta, Department of Decision Sciences, Filippo Trentini, Dondena Center) based on the analysis of consolidated data produced by the Prevention - Infectious Diseases Organizational Unit of the DG Welfare of the Lombardy Region and by infectivologists and virologists of ATS, IRCSS, ASSTs in Lombardy.
 
In the study, which consolidates the analyses produced by the team as of February 2020, the epidemiological data of SARS-CoV-2 in Lombardy were retrospectively analyzed and the trend of transmissibility (Rt) up to March 9, 2020 was also evaluated.
 
These are some of the key findings:
 
More than 500 SARS-CoV-2-positive patients, with a median age of 69 years, reported a symptom onset date prior to Patient 1 notification (February 20, 2020). By that date, SARS-CoV-2 virus was already circulating in at least 222 of the 1,506 Lombardy municipalities (14.7%).
 
The reproduction number (R0) in the 12 provinces of Lombardy was estimated at values ranging from 2.6 in Pavia to 3.3 in Milan and Brescia. The serial interval, which approximates the time between generations of cases, was estimated to be 6.6 days on average. This implies that the time to case doubling was much less than 1 week.
 
These results suggest that the rapid growth of patients with COVID-19 that caused pressure on the Lombardy health care system in the period immediately after the identification of Patient 1 can be attributed to the high transmissibility of the virus and the widespread, silent transmission of the pathogen that occurred between January and mid-February.
 
A decrease, even a marked one, in the transmissibility (Rt) of SARS-CoV-2 was observed in all provinces of Lombardy in the period following the identification of patient 1, probably due to the growing concern in the population and the progressive introduction of restrictive control measures. However, Rt remained strictly greater than 1 until March 9, 2020, in all provinces except Lodi, where the establishment of the red zone in the Codogno area probably played a key role in controlling the epidemic by decreasing Rt to values of about 1 before the establishment of the national lockdown.
 
Danilo Cereda et al. “The early phase of the COVID-19 epidemic in Lombardy, Italy.”
Epidemics, Volume 37, 2021, DOI: https://doi.org/10.1016/j.epidem.2021.100528.
 

by Ezio Renda