Between September 3, 2019 to March 2020, blood was taken from healthy trial volunteers from across several Italian regions who were enrolled in a lung cancer screening trial. Upon later analysis it was found the blood from many participants had the “unexpected detection of SARS-CoV-2 antibodies”, strongly suggesting the virus was in circulation in Italy as early as September 2019, approximately 5 months before the first official reported case of COVID-19 in February 2020.
The National Cancer Institute (INT) of the Italian city of Milan published their data in Tumori Journal and it is reported to show that 11.6% of 959 healthy volunteers enrolled in a lung cancer screening trial had developed SARS-CoV-2 antibodies well before February 21, 2020 when their first official COVID-19 patient was recorded.
Italian researchers told Reuters in March 2020 that they reported a higher than usual number of cases of severe pneumonia and flu in Lombardy in the last quarter of 2019 [1]
Italy had a bad 2019 flu season (late 2019), and so did the US. [1]
Tests of the US blood supply indicates the SARS-CoV-2 antibodies were present in December 2019.