SARS-CoV-2 introductions and early dynamics of the epidemic in Portugal

Background Genomic surveillance of SARS-CoV-2 in Portugal was rapidly implemented by the National Institute of Health in the early stages of the COVID-19 epidemic, in collaboration with more than 50 laboratories distributed nationwide. Methods By applying recent phylodynamic models that allow integration of individual-based travel history, we reconstructed and characterized the spatio-temporal dynamics of SARS-CoV-2 introductions and early dissemination in Portugal. Results We detected at least 277 independent SARS-CoV-2 introductions, mostly from European countries (namely the United Kingdom, Spain, France, Italy, and Switzerland), which were consistent with the countries with the highest connectivity with Portugal. Although most introductions were estimated to have occurred during early March 2020, it is likely that SARS-CoV-2 was silently circulating in Portugal throughout February, before the first cases were confirmed. Conclusions Here we conclude that the earlier implementation of measures could have minimized the number of introductions and subsequent virus expansion in Portugal. This study lays the foundation for genomic epidemiology of SARS-CoV-2 in Portugal, and highlights the need for systematic and geographically-representative genomic surveillance. Plain language summary Analysing SARS-CoV-2 genetic material and how it changes over time can help us understand how the virus spreads between countries and determine the impact of control measures. In this study, we investigated SARS-CoV-2 transmission and evolution in the early stages of the COVID-19 pandemic in Portugal. In particular, we reconstructed the routes and timeliness of viral introductions into the country and assessed the relative contribution of each introduction in terms of how the epidemic evolved over time. We detected at least 277 independent introductions, mostly from European countries (namely the United Kingdom, Spain, France, Italy, and Switzerland), which were consistent with the countries with the highest connectivity with Portugal. This study reflects an unprecedented effort in the field of the infectious diseases in Portugal, highlighting the need for systematic and geographically-representative surveillance to aid public health efforts to control the virus. Borges et al. study the dynamics of SARS-CoV-2 in Portugal during 2020. The authors use phylodynamic modelling to infer patterns of SARS-CoV-2 introduction into Portugal and its subsequent spread, providing a comprehe