A comparative study between the incidence and epidemiological features of Influenza-Like Illness and laboratory-confirmed COVID-19 cases in the Italian epicenter (Lombardy)

Grosso, Francesca; Castrofino, Ambra; Del Castillo, Gabriele; Galli, Cristina; Binda, Sandro; Pellegrinelli, Laura; Cereda, Danilo; Tirani, Marcello; Gramegna, Maria; Castaldi, Silvana; Pariani, Elena

INTRODUCTION: In Lombardy, the influenza surveillance system relies on sentinel physicians that weekly report data on the number of influenza-like-illness (ILI) and a part of them also collect oropharyngeal samples for virologic analyses. This study aims at comparing the ILI incidence of 2019-2020 influenza season with the incidence of COVID-19 cases in order to better understand the current epidemic and to evaluate whether the implementation of ILI surveillance system could succeed in early detection and monitoring of COVID-19 diffusion. METHODS: The distribution of ILI cases in the seasons 2017-2018, 2018-2019 and 2019-2020 was taken in consideration and the curve trends were compared and analyzed according to geographical areas, age groups and time differences. RESULTS: The curve trends presented a similar pattern up to the 9(th) week; in fact, a reduction in the ILI incidence rate was observed in the 2017-2018 and 2018-2019 season but in the 2019-2020 an increase in the reported ILI emerged. The relation between the numbers reported by 2019-2020 ILI surveillance and those reported for COVID-19 is supported by the curve trends, the correspondence between age groups, the correspondence by geographical location, and also by the results of the nasopharyngeal swab tests performed. DISCUSSION: The influenza surveillance system is an effective tool for early detection of COVID-19. It may provide timely and high-quality data evaluating the SARS-CoV-2 burden among population with ILI. Implementation of the system has to be prioritized in order to identify any future novel respiratory pathogen with pandemic potential.