SARS-CoV-2 has been circulating in northern Italy since December 2019: evidence from environmental monitoring

Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) is responsible for the coronavirus disease COVID-19, a public health emergency worldwide, and Italy is among the world’s first and most severely affected countries. The first autochthonous Italian case of COVID-19 was documented on February 21. We investigated the possibility that SARS-CoV-2 emerged in Italy earlier than that date, by analysing 40 composite influent wastewater samples collected - in the framework of other wastewater-based epidemiology projects - between October 2019 and February 2020 from five wastewater treatment plants (WTPs) in three cities and regions in northern Italy (Milan/Lombardy, Turin/Piedmont and Bologna/Emilia Romagna). Twenty-four additional samples collected in the same WTPs between September 2018 and June 2019 were included as blank samples. Viral concentration was performed according to the standard World Health Organization procedure for poliovirus sewage surveillance. Molecular analysis was undertaken with both nested RT-PCR and real-rime RT-PCR assays. A total of 15 positive samples were confirmed by both methods. Of these, 8 were collected before the first autochthonous Italian case. The earliest dates back to 18 December 2019 in Milan and Turin and 29 January 2020 in Bologna. Samples collected in January and February in the three cities were also positive.

Our results demonstrate that SARS-CoV-2 was already circulating in northern Italy at the end of 2019. Moreover, it was circulating in different geographic regions simultaneously, which changes our previous understanding of the geographical circulation of the virus in Italy. Our study highlights once again the importance of environmental surveillance as an early warning system, to monitor the levels of virus circulating in the population and identify outbreaks even before cases are notified to the healthcare system.