The NHS app will only have a “limited effect” on containing the coronavirus because too many people are waiting more than 24 hours for their test results, a Government adviser has warned.

Dr David Bonsall, an expert in virology and infectious diseases at Oxford University, said ministers should consider looking at alerting app users when someone reports Covid-19 symptoms rather than from a test result if test times continue to lag.

Earlier versions of the NHS app did send out ‘amber’ warning alerts when users first reported Covid-19 symptoms, before they had had a positive test.

However, that system was dropped in favour of only triggering alerts after test results, over fears it would cause too many people to self isolate unnecessarily and eventually lead to people ignoring the app.

However, official figures show just one in three people who turn up for a coronavirus test in England are getting results within 24 hours.

This affects the NHS app, which is designed to log when people come into contact close enough to spread the coronvirus, via Bluetooth on their smartphones.

Dr Bosnall said he felt the best system was still one in which alerts were triggered via fast test results, but said if testing continued to lag behind the 24-hour target ministers should consider bringing in symptom-triggered alerts.

He added: “The counter arguments and behavioural arguments need to be heard, but when you are without a paddle you have to think of something.”

When a user tests positive for coronavirus, the app sends an alert to all the people they have had contact with within an infectious time period, telling them to self isolate and get a test.

Dr Bonsall, who is part of the Oxford team advising the Government on the app, told The Telegraph that roughly half of coronavirus infections took place before people showed any symptoms so it is vital people are warned to isolate as soon as someone they have had contact with tests positive.

Dr Bonsall said: “It (the drop in tests being turned round in 24 hours) means [the app] will have a limited effect and that test and tracing has a limited effect because you need to find people before they infect others.

“Testing and contact tracing can do one of two things: it can either be faster than the virus and stop it, or it can be slower than the virus and you can watch it spread. That is the cruel dichotomy that you have to work with.”

He also said the NHS Covid-19 app, which will be released on September 24, could be key in driving down infections among tech-savvy teenagers and young adults, who have been at head of the recent surge in cases.

Dr Bonsall also warned that the Government needed to focus on boosting access to walk-in test sites, arguing that home deliveries were far less likely to deliver results in under 24 hours.

The NHS app is due to launch following a series of setbacks and delays.

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