Was severe SARS-CoV-2 substantially spreading in Northern Italy before its first detection in February 2020? An evaluation of pneumonia-associated hospitalization trends from September 2014 to February 2020.

Retrospective studies identified SARS-CoV-2 worldwide circulation as early as late 2019. In Italy, the first autochthonous COVID-19 case was diagnosed in a Northern Region on 20 February 2020, raising the question whether high numbers of COVID-19 pneumonia cases were previously undetected. We explored whether unusual increases in hospitalizations for pneumonia occurred from October 2019 to February 2020 in Italy, particularly in Northern Regions. We analysed the Italian National Hospital Discharge Records with pneumonia ICD-9-CM codes from 2014 to 2020. Trend analysis and generalized linear models with negative binomial distribution were applied to compare observed pneumonia trends in the study period with previous years. Analyses were stratified by major regions (NUTS1) and provinces. During the study period, 2 501 074 hospitalizations were coded as pneumonia. No unusual increases of all hospitalizations associated to pneumonia were observed until mid-February 2020. Hospitalizations with viral pneumonia ICD9-CM codes were negligible until the end of January 2020, with a significant increase in two provinces of Lombardy Region 1-2 weeks before the first autochthonous COVID-19 case. Our analysis showed that a small increase in viral pneumonia hospitalizations in Northern Italy only in the weeks immediately preceding the first locally acquired SARS-CoV-2 case in two provinces of Lombardy. This excludes large-scale circulation in the last months of 2019 and in January 2020. Given the mild 2019-2020 influenza season and lower pneumonia hospitalization burden, the initial increase could have been interpreted as a fluctuation as it did not determine an overall excess case-load of pneumonia hospitalizations.