A delayed update to the NHS Covid-19 contact tracing app resulted in fewer users receiving notifications to quarantine. The app was recently updated to lower the “risk threshold” so that people receive a notification after coming into contact with a person who tested positive for Covid-19 when they are most infectious. Photo credit: Dan Kitwood, Getty Images
An exposure notification app developed by the UK’s National Health Service failed to alert users to quarantine after coming into contact with people who tested positive for Covid-19.
The app, based on Apple and Google’s exposure notification framework, detects users’ proximity to others using a Bluetooth signal from their smartphones. People receive an alert to quarantine if they have recently come into contact with someone who reported testing positive for Covid-19. But a missed update to the app resulted in a “shockingly low” number of people being told to self-isolate, according to the Sunday Times.
Before the app launched in September, an “infectiousness” measure was added to the algorithm, to account for the fact that people are most infectious shortly after symptoms appear. Users of the app are asked to input when their symptoms first began.
After this update, the “risk threshold” for the algorithm was supposed to be lowered so that people who spent just three minutes in contact with someone who was highly infectious would receive an alert to quarantine. But it was never updated, meaning people had to spend 15 minutes near an infectious person, or 40 minutes near a pre-symptomatic person to receive an alert, according to the Guardian.
In a Thursday blog post, two officials with the Department of Health and Social Care who were involved in the app’s development confirmed that the “risk threshold” was supposed to be lowered, but the change did not take place at that time. A recent update to the app should correct this.
“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,” they wrote.
The changes should also make the location approximation more accurate by using both time data and signal strength to estimate the distance between two devices.
The update will also remove “possible exposure” notifications, which confused app users who received an alert that they had come into contact with someone who tested positive for Covid-19, but were not instructed to quarantine.
“Often, the NHS COVID-19 app would establish someone was not a high-risk to you and so wouldn’t alert you,” according to the blog post. “This was confusing for app users who would receive a ‘possible exposure’ message.”
Now, people will only receive a notification if they are supposed to quarantine.
As with other tools built using Apple and Google’s framework, public health officials cannot view users’ locations and recent contacts. This is important from a privacy perspective, but it also limits the apps’ usefulness for traditional contact tracing.
While uptake of Apple and Google’s framework has been slow across the U.S., the NHS app has more than 19 million users, according to the agency.
