Community transmission of SARS-CoV-2, understood as the presence of established transmission chains, could have started as early as January 2020 in some areas of Europe and the United States according to a modeling study published in Nature to which the ISI Foundation in Turin and Fondazione Bruno Kessler in Trento have contributed. The analysis shows that the virus was introduced in several areas of Europe and the United States before the end of January 2020 and its spread in the first three months of 2020 was largely undetected, due to the limited ability to identify and test suspicious cases during that time. Only about 1-3% of infections were detected in these areas as of March 2020.