PEOPLE possibly infected with coronavirus need to be notified within 24 hours or risk the crucial contact tracing technology being ineffective, a key scientist advising the Government on its NHS Covid-19 app said.

Professor Christophe Fraser, senior group leader in pathogen dynamics at University of Oxford Big Data Institute, said time was a main factor in the app helping contain the spread of coronavirus.

The smartphone download is being trialled on the Isle of Wight before its anticipated roll-out across the rest of the country later this month.

It is deemed crucial to the Government’s contact-tracing programme, which will require smartphone users to download the app and share data the moment they test positive or display signs of Covid-19, with the app sending a notification to all mobile phone users in recent proximity to them advising them to self-isolate.

Environment Secretary George Eustice said at the daily Downing Street press conference on Friday that the app would become “highly effective” if used by 60per cent of the population, adding: “But even at lower rates than that it would be effective.”

But Prof Fraser said speed was vital to ensure the technology’s efficacy, meaning those possibly infected needed to be notified within 24 hours of symptoms emerging in anyone with whom they have been in contact.

He told Sky News: “A 72-hour delay really means that you’re having very little impact on the epidemic.

“A 48-hour delay is pretty bad.

“You really need to be getting the information across in 24 hours.”

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Prof Fraser said data modelling suggested warnings being sent out within 24 hours could “make the difference between an epidemic starting to resurge towards the second wave or not”.

Concerns have also been raised about how effective the app will be when many elderly people do not have smartphones.

Mr Eustice said: “We do recognise that some elderly people may not have a mobile phone ... but many will have family who will be able to support them to get such an app.”

NHS England medical director Professor Stephen Powis added: “I know from my experience in healthcare not to underestimate the benefits that technology such as smartphones can bring to the older generation and many do indeed use this technology.

“It’s absolutely correct that the app will not be the only way in which we will be able to monitor and track down contacts of people who have coronavirus going forward.

“It will be an important part but not the only part.”

It came as the Financial Times reported NHSX, the health service’s digital technology arm behind the NHS Covid-19 app, sanctioned working with industry giants Apple and Google to develop a second app.

Prof Fraser told Sky News: “There are advantages to the Google/Apple system.

“They operate the operating system and it’s important to get the highest possible uptake.

“It’s important that people feel confident and feel that the privacy concerns are being taken seriously and that the system collects as little information as it needs to function.”