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The algorithm and risk score

The app uses Bluetooth signal strength between devices to estimate the distance between 2 close devices.

Signal strength between 2 close devices is measured every 3.6 minutes (on average).

Signal strength depends on a number of external factors, such as where the phone is on the body and the surrounding environment.

While a Bluetooth signal continues to be detected between 2 devices (an encounter), Application Programming Interface (API) Mode 2 gathers signal strength measurements that are transmitted by another user鈥檚 device approximately 4 seconds every 3.6 minutes (on average).

A range of sample measurements relating to the signal strength are gathered by the API and used together to estimate of distance. Sets of measurements within an encounter are shuffled to ensure anonymity and then assessed together to build an estimate of the distance between the 2 devices during this period.

For each encounter, a score is calculated as follows:

• estimated within 1m of the other device: total time spent within 1m
• estimated 1m or beyond from the other device: sum of [total time at each distance / distance squared]

These scores are summed across the interactions someone has had in a single day with the contact who has tested positive. The app does not look at interactions across multiple days as it uses new anonymous IDs every day in order to protect the privacy of users.

Finally, the total score is multiplied by a factor recognising infectiousness (of the individual testing positive) on the day of the encounter.

Risk threshold and notifications

When an app user enters a positive test result in the app, they are asked whether the app can notify other app users they may have passed the virus on to.

The algorithm uses the date of symptom onset to calculate who they are most likely to have infected. If they said they cannot remember when their symptoms started, the app will record it as the same day as their test date. The algorithm looks at the daily risk for every app user who they came into contact with from 2 days before symptoms first appeared, up to a maximum of 10 days after symptoms first appeared.

If an app user鈥檚 risk score on any of these days crosses the risk threshold it is identified as high-risk and they will be notified and directed to further guidance.

For the purposes of contact tracing, a high-risk encounter is classed as one where an individual has been within 2 metres of someone who has tested positive for coronavirus (COVID-19) for at least 15 minutes. The risk threshold for the app has been set to identify high-risk encounters based on this principle along with infectiousness (of the individual testing positive) on the day of the encounter; balancing the need to slow the rate of infection by identifying high-risk encounters whilst mitigating the risk that those at lower risk may be notified to self-isolate.

This threshold can be changed as the app is updated, more data is available about the transmission rate of the virus, or due to behavioural factors.

The algorithm has been optimised by scientific experts, based on the functionality provided by V1.6 of Google鈥檚 and V2.0 of Apple鈥檚 API.

Mobile or wifi data

The app requires an internet connection at regular intervals throughout the day. This is to download a list of anonymous IDs of people who have recently tested positive for COVID-19.

The amount of data used will vary from day to day, depending on how many COVID-19 cases there are. The app team will be carefully monitoring data usage to make sure it remains at a reasonable level.

Major phone operators in England and Wales, including Vodafone, Three, EE, O2 (including giffgaff and Tesco mobile), Sky and Virgin, are supporting NHS COVID-19 app users by 鈥榸ero-rating鈥� data charges for all in-app activity. This means customers will not be charged for data when using the in-app functions.

Don鈥檛 worry if your internet connection is unreliable. Alerts and notifications from the app will be queued and sent to you as soon as you鈥檙e back online.

How we know the app works

The NHS COVID-19 app has been shown to help reduce the spread of COVID-19.

Leading scientists at the University of Oxford and University of Warwick have estimated in a (February 2023) that the app鈥檚 contact tracing functionality prevented around 1 million cases, 44,000 hospitalisations and 9,600 deaths during its first year alone.

This builds on previous analysis by the Alan Turing Institute, University of Warwick and University of Oxford regarding the app鈥檚 effect from its launch in September 2020 to the end of December 2020. , showed that for every 1% increase in app users, the number of COVID-19 cases in the population was reduced by up to 2.3%.