Millions are set to benefit from reduced waiting times and improved public services, as technology experts, advocates, and academics come together to shape the government's new 'digital centre.

How likely is it to become infected by SARS-CoV-2 after being exposed? Virtually everyone has 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-metre 15-minute guideline4,5, despite limited evidence supporting this threshold. Here we analysed 7 million contacts notified by the NHS 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 similar risk to 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. While most exposures were short (median 0.7 hours, IQR 0.4-1.6), transmissions typically resulted from exposures lasting one hour to several days (median 6 hours, IQR 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 a new pathogen emerging.

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