Media release

From: Springer Nature

Retracing the steps of the COVID-19 pandemic

Community transmission of SARS-CoV-2 may have begun as early as January 2020 in some areas of Europe and the USA according to a modelling study published in Nature. The analysis indicates that the virus was introduced to several areas within Europe and the USA by January 2020 and its spread in the first three months of 2020 was largely undetected, owing to limited testing for infection. Only around 1–3% of infections were being detected in these regions by March 2020, highlighting areas for improvement to guide future pandemic-response strategies.

Nearly two years into the COVID-19 pandemic there is still uncertainty about the timeline of introductions and local transmission of SARS-CoV-2 across the world. In the early stages of the pandemic, some countries limited testing to symptomatic individuals with travel links to China, which contributed to the undetected transmission of the virus. To improve our understanding of the dynamics of the evolution of the COVID-19 pandemic in the USA and Europe, Alessandro Vespignani and colleagues modelled the plausible introduction and dispersal of the virus during the first wave of the pandemic in 2020.

Their analysis indicates that in early March 2020, around 9 in 1,000 infections were detected in the USA and 35 in 1,000 infections were detected in Europe. For the week commencing 8 March, there were 2 and 17 reported cases per day in the USA and Europe, respectively, whereas the model estimates that there may have been nearly 2,000 and 4,000 new cases daily, respectively. Estimates of the timing of introduction vary by country or state; the onset of transmission is predicted to have begun by 26 January in California and 2 February in New York state, but possibly as early as 6 January in Italy. The authors cannot rule out introduction and transmission events as early as December 2019, although the probability is very small.

The results highlight the potential extent of the early onset of local transmission in the USA and Europe that went largely undetected. More widespread testing and a broader testing criteria may have enabled earlier detection and interventions to prevent the spread of SARS-CoV-2, the authors suggest. They conclude that the techniques and modelling approaches used in this study may be helpful to inform future response strategies for emerging viruses.