The early phase of the COVID-19 epidemic in Lombardy, Italy
Danilo Cereda,
Mattia Manica,
Marcello Tirani,
Francesca Rovida,
Vittorio Demicheli,
Marco Ajelli,
Piero Poletti,
Filippo Trentini,
Giorgio Guzzetta,
Valentina Marziano,
Raffaella Piccarreta,
Antonio Barone,
Michele Magoni,
Silvia Deandrea,
Giulio Diurno,
Massimo Lombardo,
Marino Faccini,
Angelo Pan,
Raffaele Bruno,
Elena Pariani,
Giacomo Grasselli,
Alessandra Piatti,
Maria Gramegna,
Fausto Baldanti,
Alessia Melegaro,
Stefano Merler
Affiliations
Danilo Cereda
Directorate General for Health, Lombardy Region, Milano, Italy
Mattia Manica
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Marcello Tirani
Directorate General for Health, Lombardy Region, Milano, Italy; Health Protection Agency of the Metropolitan Area of Milan, Milano, Italy; Correspondence to: Directorate General for Health, Lombardy Region, Piazza Città di Lombardia 1, 20124 Milano, Italy.
Francesca Rovida
Molecular Virology Unit, Fondazione IRCCS Policlinico San Matteo, Pavia, Italy; Department of Clinical, Surgical, Diagnostics and Pediatric Sciences, University of Pavia, Pavia, Italy
Vittorio Demicheli
Health Protection Agency of the Metropolitan Area of Milan, Milano, Italy
Marco Ajelli
Laboratory for Computational Epidemiology and Public Health, Indiana University School of Public Health, Bloomington, United States
Piero Poletti
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Filippo Trentini
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy; Carlo F. Dondena Centre for Research on Social Dynamics and Public Policy, and CovidCrisisLab, Bocconi University, Milano, Italy
Giorgio Guzzetta
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Valentina Marziano
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy
Raffaella Piccarreta
Carlo F. Dondena Centre for Research on Social Dynamics and Public Policy, and CovidCrisisLab, Bocconi University, Milano, Italy; Department of Decision Sciences, Bocconi University, Milan, Italy
Antonio Barone
Regional Agency for Innovation and Procurement, Milano, Italy
Michele Magoni
Epidemiology Unit, Health Protection Agency of Brescia, Brescia, Italy
Silvia Deandrea
Department of Hygiene and Preventive Medicine, Health Protection Agency of Pavia, Pavia, Italy
Giulio Diurno
Directorate General for Health, Lombardy Region, Milano, Italy
Massimo Lombardo
ASST Lodi, Lodi, Italy
Marino Faccini
Health Protection Agency of the Metropolitan Area of Milan, Milano, Italy
Angelo Pan
Infectious Diseases Unit, ASST di Cremona, Cremona, Italy
Raffaele Bruno
Department of Clinical, Surgical, Diagnostics and Pediatric Sciences, University of Pavia, Pavia, Italy; Infectious Diseases Unit, Fondazione IRCCS Policlinico San Matteo, Italy
Elena Pariani
Department of Biomedical Sciences for Health, University of Milan, Milano, Italy
Giacomo Grasselli
Department of Pathophysiology and Transplantation, University of Milan, Milano, Italy; Department of Anesthesia, Intensive Care and Emergency, Fondazione IRCCS Ca’ Granda Ospedale Maggiore Policlinico, Milano, Italy
Alessandra Piatti
Directorate General for Health, Lombardy Region, Milano, Italy
Maria Gramegna
Directorate General for Health, Lombardy Region, Milano, Italy
Fausto Baldanti
Molecular Virology Unit, Fondazione IRCCS Policlinico San Matteo, Pavia, Italy; Department of Clinical, Surgical, Diagnostics and Pediatric Sciences, University of Pavia, Pavia, Italy
Alessia Melegaro
Carlo F. Dondena Centre for Research on Social Dynamics and Public Policy, and CovidCrisisLab, Bocconi University, Milano, Italy; Department of Social and Political Sciences, Bocconi University, Milano, Italy
Stefano Merler
Center for Health Emergencies, Bruno Kessler Foundation, Trento, Italy; Correspondence to: Center for Health Emergencies, Bruno Kessler Foundation, Via Sommarive 18, 38123 Povo, TN, Italy.
Background: In the night of February 20, 2020, the first epidemic of the novel coronavirus disease (COVID-19) outside Asia was uncovered by the identification of its first patient in Lombardy region, Italy. In the following weeks, Lombardy experienced a sudden increase in the number of ascertained infections and strict measures were imposed to contain the epidemic spread. Methods: We analyzed official records of cases occurred in Lombardy to characterize the epidemiology of SARS-CoV-2 during the early phase of the outbreak. A line list of laboratory-confirmed cases was set up and later retrospectively consolidated, using standardized interviews to ascertained cases and their close contacts. We provide estimates of the serial interval, of the basic reproduction number, and of the temporal variation of the net reproduction number of SARS-CoV-2. Results: Epidemiological investigations detected over 500 cases (median age: 69, IQR: 57–78) before the first COVID-19 diagnosed patient (February 20, 2020), and suggested that SARS-CoV-2 was already circulating in at least 222 out of 1506 (14.7%) municipalities with sustained transmission across all the Lombardy provinces. We estimated the mean serial interval to be 6.6 days (95% CrI, 0.7–19). Our estimates of the basic reproduction number range from 2.6 in Pavia (95% CI, 2.1–3.2) to 3.3 in Milan (95% CI, 2.9–3.8). A decreasing trend in the net reproduction number was observed following the detection of the first case. Conclusions: At the time of first case notification, COVID-19 was already widespread in the entire Lombardy region. This may explain the large number of critical cases experienced by this region in a very short timeframe. The slight decrease of the reproduction number observed in the early days after February 20, 2020 might be due to increased population awareness and early interventions implemented before the regional lockdown imposed on March 8, 2020.
