Those "pinged" by the NHS app in England and Wales are up to four times more likely to have Covid-19 than someone who is not, research suggests.
A survey of more than 750,000 Zoe Covid Symptom Study contributors found only 2.4 per cent of fully vaccinated participants who were pinged, but felt physically normal, went on to test positive.
However, those in the same group who had one or more coronavirus symptoms were 11.7 times more likely to test positive.
Overall, those who had been alerted to self-isolate by the app had between a 3.7 to 4.0 relative risk of having the virus.
People who reported being "pinged" by the Protect Scotland app were 10 times more likely to have Covid-19 than someone who was not.
The research suggests a "large number" of people who are not infected are being asked to self isolate in the UK after being alerted by tracing apps.
Additional symptom tracking and contact tracing apps could improve accuracy and avoid the need for low-risk individuals to self-isolate with Covid-19, the study argued.
The Protect Scotland app was found to be more accurate at alerting positive-testing people compared to the English and Welsh equivalents.
The survey also found that younger groups who were told to self-isolate by an app were more likely to be positive compared to older age groups, and that unvaccinated participants were much more likely to contract the virus compared to partially and fully vaccinated users.
Other findings suggest symptom tracking and contact tracing increases the likelihood of contacting someone who has coronavirus.
Professor Tim Spector, lead scientist of Zoe Covid Study app, said: "On Thursday the Health Secretary announced fully jabbed Covid contacts won't need to self-isolate from August 16.
"While I welcome efforts to address the 'pingdemic', cases are still very high and our research shows that whatever your vaccination status, if you have one of possible 20-plus Covid symptoms recognised by Zoe - not just the classic three - you should be cautious, self isolate and get a test.
"Which makes it even more important for the Government to expand the symptom list to help people understand all the symptoms of Covid-19.
"This rapid analysis is proof that the Zoe Covid Study is in a unique position to gather data and continue to monitor the effectiveness of contact tracing apps.
"With access to nearly a million weekly contributors we can ask questions at a huge scale and get answers fast."
