The number of people being told to self-isolate by the NHS Covid app has fallen dramatically.

A total of 395,971 alerts were sent in the week to July 28 telling people in England and Wales that they had been in close contact with someone who had tested positive for coronavirus.

This is down 43% on the previous week, when 690,129 alerts were sent on the app, Government figures show.

There has also been a big decrease in the number of check-ins to venues using the app.

There were 2.4 million check-ins in the week to July 28 in England and Wales – down 65% on the previous week.

Experts said the main reason for the sharp decrease in alerts is the fall in virus cases, but some acknowledged there could also be people simply not using the app anymore.

On Monday it was announced that the app was being updated so fewer contacts will be instructed to isolate after an increase in people being pinged since lockdown restrictions ended – a so-called “pingdemic”.

Health and Social Care Secretary Sajid Javid said the “logic” behind the app was being tweaked, although the sensitivity and risk threshold will remain unchanged.

Instead of checking contacts for five days before a positive test, the app will only go back two days.

Paul Hunter, professor in medicine at the University of East Anglia, said: “The primary reason for the fall in pings is the big drop in cases.

“There may have been some additional impact of people disabling that aspect of the app but the main reason is the drop in cases.”

Professor Christophe Fraser, scientific adviser to the Test and Trace programme, said a question to be considered now is to what extent the raft of alerts sent in recent weeks telling users to self-isolate had contributed to a fall in cases.

The senior group leader in pathogen dynamics at Oxford University’s Nuffield Department of Medicine said: “The number of pings is determined by the number of cases, the number of users, and the number of risky contacts made by users.

“The reason the number of pings has gone down is the number of cases has gone down.

“One question under investigation is to what extent the ‘pingdemic’ contributed to the decline in cases.”

But Dr Philip Scott, chair of the health and care executive at BCS, The Chartered Institute for IT, claimed the app had led to “so many pointless alerts” before the tweak, which came into effect from August 2.

The reader in health informatics at the University of Portsmouth said: “People are either uninstalling it or just not running it, according to the data that shows venue check-in plummeting.

“Overall the app has served a purpose, but there is a need to challenge why its policy basis, of legal enforceability, was different to Test and Trace and why the alerting algorithm was not the same.

“This is one example that shows why better professional IT standards should be more closely followed when it comes to apps, and algorithms, that affect are daily lives.”

Under the Test and Trace system, a contact is considered a person who has been close to someone who has tested positive any time from two days before the person developed symptoms, or from two days before the date their positive test was taken.

While there is a legal duty in England for people to self-isolate if they test positive or are contacted by NHS Test and Trace, this does not extend to notifications via the app, although the Government strongly advises people to do so.

Mike Tildesley, a member of the Scientific Pandemic Influenza Modelling group (Spi-M) advising ministers, described the app as “incredibly useful”, despite large numbers of people being asked to isolate.

He told Sky News: “I know there have been some challenges in terms of, particularly at the moment, the so-called ‘pingdemic’, but in terms of being able to detect contact, it has been extremely valuable.

“Obviously the challenge with that is that a lot of people are going into isolation and over the last few days the app has been made less sensitive.”

Dr Tildesley said there is a worry that if too many people are pinged, fewer may be willing to comply, but he added that the tweak will “hopefully guarantee higher levels of compliance”.