Digital measurement of SARS-CoV-2 transmission risk from 7 million contacts

Luca Ferretti (), Chris Wymant, James Petrie, Daphne Tsallis, Michelle Kendall, Alice Ledda, Francesco Di Lauro, Adam Fowler, Andrea Di Francia, Jasmina Panovska-Griffiths, Lucie Abeler-Dörner, Marcos Charalambides, Mark Briers and Christophe Fraser ()
Additional contact information
Luca Ferretti: University of Oxford
Chris Wymant: University of Oxford
James Petrie: University of Oxford
Daphne Tsallis: Zühlke Engineering Ltd
Michelle Kendall: University of Warwick
Alice Ledda: UK Health Security Agency
Francesco Di Lauro: University of Oxford
Adam Fowler: University of Oxford
Andrea Di Francia: UK Health Security Agency
Jasmina Panovska-Griffiths: University of Oxford
Lucie Abeler-Dörner: University of Oxford
Marcos Charalambides: The Alan Turing Institute
Mark Briers: The Alan Turing Institute
Christophe Fraser: University of Oxford

Nature, 2024, vol. 626, issue 7997, 145-150

Abstract: Abstract How likely is it to become infected by SARS-CoV-2 after being exposed? Almost everyone wondered about this question during the COVID-19 pandemic. Contact-tracing apps1,2 recorded measurements of proximity3 and duration between nearby smartphones. Contacts—individuals exposed to confirmed cases—were notified according to public health policies such as the 2 m, 15 min guideline4,5, despite limited evidence supporting this threshold. Here we analysed 7 million contacts notified by the National Health Service COVID-19 app6,7 in England and Wales to infer how app measurements translated to actual transmissions. Empirical metrics and statistical modelling showed a strong relation between app-computed risk scores and actual transmission probability. Longer exposures at greater distances had risk similar to that of shorter exposures at closer distances. The probability of transmission confirmed by a reported positive test increased initially linearly with duration of exposure (1.1% per hour) and continued increasing over several days. Whereas most exposures were short (median 0.7 h, interquartile range 0.4–1.6), transmissions typically resulted from exposures lasting between 1 h and several days (median 6 h, interquartile range 1.4–28). Households accounted for about 6% of contacts but 40% of transmissions. With sufficient preparation, privacy-preserving yet precise analyses of risk that would inform public health measures, based on digital contact tracing, could be performed within weeks of the emergence of a new pathogen.

Date: 2024
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DOI: 10.1038/s41586-023-06952-2

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