The NHS Covid-19 app in England and Wales is being tweaked so fewer contacts will be advised to self-isolate following a close contact with a positive case.
The app will look for contacts two days prior, rather than five, when someone without symptoms tests positive.
Health Secretary Sajid Javid said it will reduce self-isolation disruption and protect those at risk.
The government is urging the public to continue using the app.
Scientists have estimated the app prevented up to 2,000 cases per day in the three weeks to 21 July when the number of self-isolation alerts – known as “pings” – rose by over 70,000 to a new record of 689,313.
And their analysis, which assumed 60% compliance with instructions to self-isolate, showed another 50,000 Covid cases were prevented by including chains of transmission.
This was estimated to have prevented 1,600 hospitalisations, the Department of Health and Social Care (DHSC) said.
“We want to reduce the disruption that self-isolation can cause for people and businesses, while ensuring we’re protecting those most at risk from this virus.”
It said the app reduced the spread of Covid-19 by around 4.3% each week. And for every 200 to 250 tests entered and shared in the app, one person is prevented from being hospitalised from the virus.
Around 40% of the eligible population regularly use the app and around 50% of all reported tests being inputted, the DHSC said.
From 16 August, fully vaccinated contacts in England will be exempt from isolation and instead will be advised to take a PCR test.
Those who are not fully vaccinated will still be required to isolate, and it will remain a legal obligation to isolate if you test positive.
Trust still essential
There was speculation when Sajid Javid became the new health secretary just as the word “pingdemic” was entering the language that he would order changes to make the app less sensitive.
That was ruled out – but it seems Mr Javid asked what else could be done and was advised that the option unveiled on Monday would be wiser.
Cutting the period the app hunts back through the close contacts of an asymptomatic person before a positive test from five days to two could mean a substantial fall in the numbers sent into isolation.
There has been huge pressure for change from businesses which have seen their operations disrupted by thousands of pings – but at the same time the team behind the app believes it now has strong evidence that it is working.
The government will hope it has found a middle way, reducing the burden on businesses and making it less likely that people will delete the app while still alerting those in most danger of becoming infected with the virus and passing it on.
But as with so much about this project this is an experiment in human behaviour because for the app to work, people have to trust it.
Announcing the change, Mr Javid said: “We want to reduce the disruption that self-isolation can cause for people and businesses, while ensuring we’re protecting those most at risk from this virus.
“This update to the app will help ensure that we are striking the right balance.”