Cryptic transmission of SARS-CoV-2 and the first COVID-19 wave

Nature. 2021 Dec;600(7887):127-132. doi: 10.1038/s41586-021-04130-w. Epub 2021 Oct 25.


Considerable uncertainty surrounds the timeline of introductions and onsets of local transmission of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) globally1-7. Although a limited number of SARS-CoV-2 introductions were reported in January and February 2020 (refs.8,9), the narrowness of the initial testing criteria, combined with a slow growth in testing capacity and porous travel screening10, left many countries vulnerable to unmitigated, cryptic transmission. Here we use a global metapopulation epidemic model to provide a mechanistic understanding of the early dispersal of infections and the temporal windows of the introduction of SARS-CoV-2 and onset of local transmission in Europe and the USA. We find that community transmission of SARS-CoV-2 was likely to have been present in several areas of Europe and the USA by January 2020, and estimate that by early March, only 1 to 4 in 100 SARS-CoV-2 infections were detected by surveillance systems. The modelling results highlight international travel as the key driver of the introduction of SARS-CoV-2, with possible introductions and transmission events as early as December 2019 to January 2020. We find a heterogeneous geographic distribution of cumulative infection attack rates by 4 July 2020, ranging from 0.78% to 15.2% across US states and 0.19% to 13.2% in European countries. Our approach complements phylogenetic analyses and other surveillance approaches and provides insights that can be used to design innovative, model-driven surveillance systems that guide enhanced testing and response strategies.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Air Travel / statistics & numerical data
  • COVID-19 / epidemiology*
  • COVID-19 / mortality
  • COVID-19 / transmission*
  • COVID-19 / virology
  • China / epidemiology
  • Disease Outbreaks / statistics & numerical data
  • Epidemiological Models*
  • Europe / epidemiology
  • Humans
  • Population Density
  • SARS-CoV-2 / isolation & purification*
  • Time Factors
  • United States / epidemiology