The NHS Covid-19 app has been upgraded to detect close contacts more confidently, meaning that users can now expect more high-risk notifications to be issued.
Due to the app's improved accuracy, users who are notified to self-isolate should take warnings even more seriously, the Department of Health and Social Care (DHSC) said as it announced the changes.
The Covid-19 app, which was built by the NHS to anonymously track and warn people who are at risk of having been infected by the disease in England and Wales, relies on a Bluetooth-enabled API developed by Apple and Google for health services to develop decentralised, privacy-preserving contact-tracing tools.
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Phones that are fitted with the app regularly generate random, anonymous IDs that are exchanged via Bluetooth whenever two devices that have downloaded the app come into prolonged contact. If someone later tests positive for Covid-19, they have the option to submit their results to the app, which in turn triggers alerts to contacts deemed to be at risk of infection by the algorithm.
Google and Apple have recently upgraded their API, and Gaby Appleton, NHS director of product for Test and Trace, maintained that the NHS Covid-19 app is the first in the world to integrate these improvements.
The first tweak concerns accuracy. Experts have often cited the risk of digital contact-tracing tools sending out notifications to users to self-isolate as a result of false positives, due to Bluetooth technology's lack of accuracy in measuring distance, which could undermine trust in the app.
"The latest update uses technology to better measure distance, meaning we can reduce the number of low-risk contacts notified to self-isolate without impacting the number of high-risk contacts notified to self-isolate," said the DHSC.
Bluetooth signals tend to be unreliable, bouncing off different surfaces and creating echoes, which means that the app can record radically different distances between two users, even over a short period of time. Thanks to Google and Apple's new API, the app's algorithm can now take this into account when trying to establish how far two handsets are from each other, using statistics to narrow in on where a contact actually is.
The improved algorithm was developed in collaboration with scientists from the Alan Turing Institute, and health services are now confident that the technology is highly accurate. The DHSC is, therefore, prepared to let the app issue more alerts to users than before, advising those who are at high risk of infection to self-isolate.
When a Bluetooth contact is registered by the app and followed by one of the two concerned users testing positive, some calculations need to be carried out to determine whether the interaction carried a high risk of infection. Distance, time, and the infectiousness of the person who tested positive together create an "alert trigger score", or "risk threshold", which, when it is met, triggers a notification.
If a contact lasted for a short period of time, for example, or happened at a great distance, it's unlikely that an alert will be issued. The criteria that define whether an alert should be released, however, are determined by health services and can change as the epidemic evolves.
The app's new upgrade has been designed to take stronger account of the degree of infectiousness of someone infected with the coronavirus -- the fact that, on the day that symptoms develop, the virus is much more likely to spread.
As a result, the risk threshold has been lowered, making it more likely that contact with an infected person will trigger a self-isolate command from now on.
"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," said Appleton. "We believe lowering the threshold is necessary to reduce the R rate and break the chain of transmission."
Although the NHS Covid-19 app has now been downloaded by 40% of eligible smartphone owners (19 million users) across England and Wales, the impact of the technology is still unclear. The researchers behind the tool are confident that the app will be working alongside traditional interventions to reduce infections, but key data remains undisclosed.
For example, there is no detailed information about the number of positive tests that have been submitted in the app, or the number of times the tool has been uninstalled.
Among the other improvements enabled by the latest version of Google and Apple's API is the removal of so-called "ghost notifications", which previously occurred when the app's algorithm was working to calculate a user's risk threshold. This led to notifications of "possible exposure", which many users reported receiving, leading to confusion even in the case of a low-risk contact.
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The DHSC has also confirmed that from early November, those who have downloaded the Covid-19 app will be able to continue using the tool if they travel to Scotland, Northern Ireland, Jersey and Gibraltar.
Scotland has, in effect, developed a server that will connect contact-tracing apps across the UK. When a user submits their positive test results in one of the apps, the anonymous diagnosis data will be sent to the Scottish-run server, and shared across all contact-tracing technologies in the nation.
Compatibility has already been established between Protect Scotland, StopCovid NI and Jersey Covid Alert. Once the NHS Covid-19 app and Beat Covid Gibraltar join the Scottish server, it won't be necessary to download different contact-tracing apps when travelling across the nation.
A similar effort, known as the "inter-operability gateway", launched in the EU this week, which has so far linked Germany's Corona-Warn app, Ireland's Covid tracker and Italy's Immuni, which together represent 30 million downloads. A second group of member states are expected to join the gateway from next week.
The UK, which is no longer a member of the EU, is not expected to join the bloc's inter-operability initiative.