Thread by @rowlsmanthorpe

NEW: The contact tracing app has been running for two weeks. The Saturday after it was launched it recorded 1.5m check-ins

So how many alerts has it sent about outbreaks in pubs and restaurants?

One. Just one
news.sky.com/story/coronavi…

MPs in the north of England are being told that pubs and bars are driving outbreaks. Mass closures are expected in parts of the country. Isn't this the the venue check-in system on the app is for?

I asked @ChiOnwurah for her reaction
news.sky.com/story/coronavi…

More on this. The contact tracing app has two main systems

📳 Bluetooth proximity detection
📳 QR check-ins to venues

Ministers were so committed to QR check-ins they made it ILLEGAL for pubs, bars and restaurants in England* not to display an official QR poster

*not Wales

The idea behind the QR code system is simple. If you check into a venue, then when there's an outbreak there, you can be warned about it

So why has only one alert been sent?

Health officials say the system is young and wasn't expected to be used frequently. They point to a similar app in New Zealand which has apparently been very successful

Here's what DHSC said when I asked them about the alerts

These are very reasonable points. We can add that the system is young and that the main contact tracing element appears to be working normally

But with mass closures of pubs and restaurants expected, you have to ask - if it doesn't send alerts, what is the QR system for?

{Here comes the science bit}

How do we know the app has only sent one alert? (1/3)

To send an alert, public health officials add the code for that venue to a data file, which is automatically sent to every contact tracing app several times a day

The app then checks to see if the codes on the file match any of the places where the user has checked in - this is all done locally on the phone to avoid sending a list of locations to a central server (2/3)

The source code for the app is available to inspect. And, if you're smarter than me, you can look through it and find the data file with the venue alerts on it

Here it is. Four alerts, three of which expired before the app was launched on 24 Sept (3/3)
…on-te-prod.prod.svc-test-trace.nhs.uk/distribution/r…

There were other secrets buried in the code. For instance: the text that'll get sent if there is a venue alert

Here's the first part (it's long). Note that it doesn't tell you which venue you were in. That not a bug, it's a feature - privacy protections mean it's not allowed

Rather than giving specific advice, the alert tells people to watch out for symptoms, a strategy DHSC officials describe as "warn and inform"

There is functionality for different alerts but as far as I can tell it hasn't been enabled