The claim: No country in the world has a working contact-tracing app.
Verdict: There are certainly countries in the world that would dispute that. Germany's app is up and running and India says its app has had 131 million downloads and traced 900,000 people to tell them to isolate.
At Prime Minister's Questions on 24 June, Boris Johnson said: "No country in the world has a working contact-tracing app."
He had previously challenged Labour leader Keir Starmer to name a country that has a "functional" contact-tracing app. He replied: "Germany - app working 15 June - 12 million downloads."
Contact-tracing apps are downloaded to people's smartphones and the idea is that if somebody tests positive for Covid-19, the app will be able to report whether they have been in close contact with anybody else who had downloaded the app.
The people with whom they have been in close contact can then be contacted and told to isolate themselves.
Germany has definitely launched its app - on 25 June it had been downloaded 13 million times. The population of Germany is about 83 million.
The number of downloads is clearly crucial to the success of a contact-tracing app because the more people who have it the more likely you are to detect an infection.
When the app was launched, German Chancellor Angela Merkel said: "The Corona-Warn-App is an important tool when it comes to tracing and breaking chains of infection."
Figures for how many people have been traced by the app have not yet been published.
The German health ministry declined to give a statement in response to Boris Johnson's comments, but it clearly believes its app is up and running.
'World's most downloaded'
India launched its app called Aarogya Setu on 2 April.
The Indian government describes its app as the most downloaded healthcare app in the world with around 131 million downloads.
India made it mandatory for government and private sector employees to download it. There has been some controversy about the app and the government published its source code last month following concerns about security and privacy.
Its latest tracing data was published a month ago. So far, the platform has contacted more than 900,000 users and told them to self-isolate.
The government said that of those who were recommended for testing, almost a quarter of them tested positive.
Prof Martin Hibberd from the London School of Hygiene and Tropical Medicine says there is no question that a number of countries have a working contact tracing app.
"The point is that these apps are working, but not yet reaching sufficient percentages of the population to be relied on as the sole means of contact tracing," he said.
"They are an excellent supplement to standard contact tracing though."
Prof Hibberd highlights Singapore's Trace Together app which he says has been working since March.
Singapore is now planning to distribute wearable devices for people without smartphones to increase the proportion of the population using them.
It has not been a completely smooth process though - there have been complaints that the system drains the phone battery, which has led some people to switch it off.
The country is running at about 200 new infections per day at the moment.
Marcel Salathé - an expert in digital epidemiology who advises the Swiss contact-tracing app initiative - referring to Boris Johnson's claim said: "I'm not sure what he means; Switzerland, Latvia, Italy, Germany and Denmark already have functioning proximity tracing apps, many other countries are releasing theirs in the coming days and weeks."
"How effective they are remains to be seen, but they are certainly functioning."
Low infection rate
France launched its app three weeks ago. Two million people downloaded it (although 460,000 have uninstalled it since) - according to figures announced at a government press conference.
So far, 68 people used it to say they had Covid-19 and 14 people have been traced and warned they are at risk of infection.
The French Digital Economy Minister Cedric O told the briefing that the low numbers reflected a decrease in the number of infections in France.
France has had about 500 new cases a day in the past week.
He conceded that the number of downloads was disappointing compared with Germany, but he said the app would be useful if there was a fresh spike in the number of cases.
So the French government clearly thinks its app is working, even if downloads have been somewhat low so far.