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. 2021 Dec:37:100528.
doi: 10.1016/j.epidem.2021.100528. Epub 2021 Nov 20.

The early phase of the COVID-19 epidemic in Lombardy, Italy

Danilo Cereda  1 , Mattia Manica  2 , Marcello Tirani  3 , Francesca Rovida  4 , Vittorio Demicheli  5 , Marco Ajelli  6 , Piero Poletti  2 , Filippo Trentini  7 , Giorgio Guzzetta  2 , Valentina Marziano  2 , Raffaella Piccarreta  8 , Antonio Barone  9 , Michele Magoni  10 , Silvia Deandrea  11 , Giulio Diurno  1 , Massimo Lombardo  12 , Marino Faccini  5 , Angelo Pan  13 , Raffaele Bruno  14 , Elena Pariani  15 , Giacomo Grasselli  16 , Alessandra Piatti  1 , Maria Gramegna  1 , Fausto Baldanti  4 , Alessia Melegaro  17 , Stefano Merler  18
Affiliations

The early phase of the COVID-19 epidemic in Lombardy, Italy

Danilo Cereda et al. Epidemics. 2021 Dec.

Abstract

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.

Keywords: COVID-19; Coronavirus; Epidemiology; Lombardy outbreak; Reproduction number; SARS-CoV-2; Transmission dynamics.

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Figures

Fig. 1
Fig. 1
Daily number of reported cases in each province by date of symptom onset. The inset zooms in on the initial part of the epidemic curve.
Fig. 2
Fig. 2
Maps of Lombardy region, its provinces, and the total population by municipality as of December 2019. Location of Lombardy within Italy is highlighted by a red box (upper left panel). Cumulative number of cases/100,000 inhabitants as of February 10, 2020 (upper right panel), February 20, 2020 (lower left panel), and March 9, 2020 (lower right panel) in each municipality of Lombardy.
Fig. 3
Fig. 3
Estimated net reproduction number Rt (weekly moving average) from the curve of symptom onset across different provinces of Lombardy.
See this image and copyright information in PMC

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