The emergence of a novel Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) caused a major disease outbreak that posed a threat to public health worldwide. Among the western countries, Italy was the first to experience a vast Coronavirus Disease 2019 (COVID-19) outbreak and the province of Bergamo experienced the deadliest COVID-19 outbreak in the
world.

Due to the rapid diffusion of the virus, most of the health care systems were unable to cope with the high demand of testing for the identification of all the infections.

In this paper researchers of Milan University and Mario Negri Institute Bergamo have sought to perform a cross-sectional study to assess the prevalence of SARS-CoV-2 infection in 423 workers in Bergamo province who returned to the workplace after the end of the Italian
lockdown on 5th May 2020.

The seroprevalence in the Bergamo area is 38.5%, significantly higher than that reported for most other regions worldwide. Among all the seropositive individuals, very few tested positive for SARS-CoV-2 in nasopharyngeal swabs, with a low viral load and no infective potential in cultured cells.

Was also estimated the detection performance of a rapid lateral flow immunochromatography assay (LFIA), showing a specificity and sensitivity
for IgG anti-SARS-CoV-2 of 98% and 92%, respectively.

The cumulative seroprevalence allowed to estimate that the actual diffusion of SARS-CoV-2 in Bergamo was 26-times higher than that reported from official data, indicating that the 96% of COVID-19 cases were underestimated.

Basing on these numbers, the fatality rate of COVID-19 in the Bergamo area lays around the 1%, significantly lower than the 20% from the official
data.

Finding that a subset of positive subjects reported symptoms attributable to COVID-19 in the first half of February 2020, suggests that COVID-19 may already have spread widely across Lombardy before the identification of the first locally transmitted case on 20th February 2020. At the end of the first wave of the pandemic, positive nasopharyngeal swabs are potentially not contagious, with implications for human-tohuman transmission of SARS-CoV-2 and the discharge policies of infected subjects.

LFIA was also identified as a valuable tool for rapid serologic surveillance of COVID-19 for population studies.