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Digital contact tracing uses a proximity detection mobile app to alert people who may be infected. It has been implemented for COVID-19 in the UK by the NHS, as well as in many other countries.
In a new study published in ScienceResearchers from the University of Oxford’s Institute of Pandemic Science, the University of Warwick and the UK Health Security Agency (UKHSA) analysed data from these apps and found that digital contact tracing can provide unprecedented insights into outbreak dynamics, enabling public health agencies to better monitor and analyse how outbreaks evolve.
The authors analysed anonymised data collected by the NHS COVID-19 app for England and Wales to ensure it was working properly. During the COVID-19 pandemic, the authors provided updates of many of the findings presented here to the UK government and public health authorities on a weekly basis, and daily at peak times, for situational awareness.
These results are now presented for the first time in a scientific publication, showing how the analyses were carried out over the entire pandemic period, as well as detailed analyses focusing on robustness and generating methods of broader applicability for use in pandemic preparedness.
The researchers used the app’s data to calculate the dynamics of the reproduction rate R – the average number of times each person infected with the virus passed it on to someone else – seeing changes five days earlier than other methods, providing an early warning system when the outbreak suddenly changed.
A unique feature of these data allowed the authors to determine when R was changing because of changes in contact patterns between people, and when it was changing because of a higher or lower probability of transmission to each person.
The researchers observed consistent variability in the transmission events detected by the app across day of the week and context – for example, they noticed that there were twice as many transmissions associated with brief encounters (less than half an hour) on Saturdays than on Mondays.
During the Christmas period of 2021, researchers observed:
• Peaks in transmissions due to short exposures on the two Saturdays before Christmas Day, three to five times higher than the baseline value, which were likely linked to commercial activities and social events.
• On Christmas Day, there was a decrease in the number of contacts with people, with transmissions coming from longer exposures, probably related to socialising within the household. The number of transmissions due to longer exposures was twice as high on Christmas Day as on the previous Saturday.
• Transmissions were again weighted towards short exposures on New Year’s Eve and New Year’s Day, where the number of transmissions from brief contacts was nearly six times higher than expected.
• Although these mixing patterns are well known, this is the first study to quantify the direct impact of these seasonal festivals on respiratory epidemic dynamics. Quantifying risk is essential for developing policy recommendations.
During the Euro football championship in July 2021:
• England match days showed significant spikes in both the number of exposures and the likelihood of transmission. Additional transmissions on match days accounted for 29% of all transmissions during the period.
• On 11 July 2021, when England played Italy in the final, transmissions were between six and nine times higher than would have been expected.
• Given that Euro 2021 matches were played in football stadiums across Europe, peaks in transmission on matchdays can be attributed primarily to social gatherings in homes and pubs, rather than fans attending the stadium.
Professor Christophe Fraser from the Pandemic Sciences Institute at the University of Oxford and lead researcher on the study, said: “Our work has shown that digital contact tracing frameworks, as well as reducing the spread of respiratory infections such as COVID-19, can be of great use in providing real-time information on the state of the epidemic and the nature of transmission.
“Ensuring digital systems are in place before new pathogens begin to spread rapidly is critical to preparing for future outbreaks.
“The worrying spread of highly pathogenic avian influenza among many mammalian species in the Americas should serve as a warning. We must put systems such as contact tracing in place in advance to prepare the world for future pandemics, ensuring that lessons learned from COVID-19 are put into practice rather than forgotten.”
Dr Michelle Kendall from the University of Warwick, who co-led the analysis, said: “Public health interventions that restrict population movement inevitably have socio-economic costs. It is important to measure which types of human contact do or do not promote transmission in order to weigh these costs against the harm caused by the disease.”
“We have shown that privacy-preserving data from digital contact tracing can reveal valuable information about transmission very quickly and with remarkable detail.
“We are grateful to everyone in England and Wales who has used the NHS COVID-19 app. Not only have they helped limit the spread, reduce pressure on the NHS and save lives, their anonymised data has also provided important real-time updates on the evolution of the outbreak and unprecedented insights into how a respiratory virus is transmitted.”
The study builds on this team’s previous work on digital contact tracing (collected here): proposing its use to speed up contact tracing for COVID-19 (Science 2020), assessing the number of infections prevented and lives saved in England and Wales (Nature 2021 and Nature Communications 2023), and understanding how each individual’s risk of infection depended on the duration and proximity of their exposure (Nature 2023).
The full article ‘Factors of real-time epidemic dynamics from daily digital measurements of COVID-19” is published in Science.