Since its launch on 24 September, the NHS COVID-19 app has been downloaded by more than 19 million people in England and Wales – that’s around 40 percent of eligible adults and more than any other European country.
As much as the app has already had great adoption, downloads and reviews, new technology is always being developed – and we are continuing to upgrade the app to make it even better with the latest version released today, including a world leading approach to estimating distance which improves the accuracy of self-isolation notifications.
This blog post explains why – and how - we are updating the technology the app uses to detect your risk of being exposed to coronavirus (COVID-19).
What is the Google and Apple API and how does it work?
To start, it’s useful to understand that the NHS COVID-19 app was built using Google and Apple’s built-in technology (API) to identify exposure risk. In summary, it is an ultra-secure way of using the Bluetooth in your phone to work out how close you have been to someone who has tested positive for coronavirus. The app is privacy-protecting and doesn’t know who you are or where you have been, so it can’t send your name or number to anyone (not the government, not the NHS, not the police).
The Google and Apple API measures the Bluetooth strength between your phone and that of other app users. If someone tests positive for coronavirus and agrees to share their data it looks at the set of Bluetooth data on your phone to estimate how close you were to that person, and for how long.
Along with infectiousness of the person testing positive, this creates a risk which is then compared to an ‘alert trigger score’ or ‘risk threshold’ in the app. If your encounters with this person are ‘scored’ higher than this threshold, it means you are at high risk and it will trigger an alert. The risk threshold is not static, it can be set to any level deemed appropriate, and will be kept under review and changed periodically to reflect the stage of the coronavirus pandemic.
Google and Apple have recently upgraded their API and so we are upgrading the NHS COVID-19 app to take advantage of the improvements they have developed. Thanks to the hard work of the Test and Trace app team, alongside scientists from The Alan Turing institute, we’re pleased to say that the NHS COVID-19 app is the first and only app globally to have upgraded their technology to utilise this API update (as far as we are aware today).
Increased accuracy using ‘time data'
The updated API considers ‘time data’ as well as improved ‘signal-strength’ data. By considering timing information between successive Bluetooth pings between two devices, we can better estimate change in distance, and so distance itself, in order to improve distance estimation accuracy.
When someone’s phone broadcasts a Bluetooth signal, the signal bounces off many different surfaces and creates echoes – making it very difficult to pinpoint the exact place the signal is coming from. This is where the ‘time data’ becomes useful. Using the time gaps between the different signals, you can use statistics to ‘smooth’ the random echoes and narrow in on where the person actually is. This is what the new API lets us do.
Compared to when we first launched the pilot of the app in August, this update reduces the chance of telling you to isolate when you re at low risk, without reducing the number of people at high risk who are notified to self-isolate.
An end to ‘possible exposure’ notifications
Another benefit of upgrading the app and employing new technology is we can also stop the ‘possible exposure’ or ‘near miss’ notifications which many users have received in recent weeks.
When you come close to someone who has tested positive for coronavirus, even for a short period of time, the app calculates the risk score to work out the risk to you. The issue for the NHS COVID-19 app has been that Google and Apple would ‘notify you’ that this was happening.
Just checking the data doesn’t mean you were definitely at high-risk. Often, the NHS COVID-19 app would establish someone was not a high-risk to you and so wouldn’t alert you. This was confusing for app users who would receive a ‘possible exposure’ message. On 13 October we updated our messaging to send users a follow up message informing them that they did not need to take action, however we recognise this is still an inconvenience and cause for concern for some app users.
By removing these notifications, users can have the confidence that if they have a contact notification, they know it is from the app itself and based on the most accurate information available.
An update to the ‘risk threshold’ to keep more people safe
The NHS COVID-19 app uses Bluetooth Low Energy to understand the distance, over time, between people who have downloaded the app. If someone tests positive for coronavirus, the app’s risk scoring algorithm uses this data, along with the infectiousness of the individual testing positive, to make calculations about risk, and work out who should be sent an alert. The calculation is based on a ‘risk threshold’ that can be set to any level and changed periodically to reflect the stage of the coronavirus pandemic.
The element of ‘infectiousness’ was added to the algorithm when the app launched nationally on 24 September. This takes into account that individuals are more infectious at the point at which symptoms develop. App users are asked to input when their symptoms began. Infectiousness is highest on the day that symptoms develop and decreases either side of this day.
To accommodate this, the ‘risk threshold’ was due to be lowered, but this change did not take place at that time. The updated version of the app addresses this by lowering the threshold at which users are deemed to be at risk of having caught the virus and alerted to self-isolate. The threshold was due to move from 900 to 180, but because we have a new statistical algorithm taking advantage of improved distance estimation, we are now lowering it to 120.
The update to the risk threshold is expected to increase the number of people asked to self-isolate by the app, having been in close contact with someone who has tested positive. We believe lowering the threshold is necessary to reduce the R rate and break the chain of transmission.
Remembering why we’re doing this
We haven’t forgotten what this app is for. It’s a tool to help control the spread of the virus, save lives and protect the economy by alerting those who need to, to isolate in order to break the chain of transmission.
This is certainly not the last upgrade we will make, and we will continue to improve and iterate both the user experience of the app and the technology behind it to ensure we have the best possible product in the fight against coronavirus. We would urge all those who can to download and use this product – by doing so you will be helping us all to protect our loved ones and communities as we enter winter.