Thread by @peripatetical

About how the Australian COVIDSafe app helps with contact tracing. A thread.
This is from the perspective of an epidemiologist - I don't have any expertise on the technical or privacy aspects.
health.gov.au/resources/apps…

Contact tracing is a manual process. The local state/territory health department routinely calls up people that have been diagnosed with COVID once they are notified.

The first step is to define the infective period - currently defined as 48 hours before symptoms developed until the time the case is called and advised to isolate.

The next step is to work out who the close contacts are. This is defined as 15 minutes face to face contact, or 2 hours in the same room as the case. There's no magic about 14 vs 16 minutes, but a line has to be drawn somewhere.

This is usually approached systematically starting from the most obvious. Who is in your household? Where do you work? Have you had face to face meetings with anyone? Who else have you been with? Meals? Car trips?

Then the public health unit calls all the people you listed to confirm that they were in the place that contact had occurred and to advise them to quarantine. For work contacts, this often requires getting phone numbers from employers.

After that, it gets difficult. Many interactions you may have are transient where the risk of transmission is minimal. But as an example, you wouldn't be able to name people you sat next to on public transport.

When you rely on recall, it's easy to miss someone. Was that meeting 4 or 5 days ago? Was that conversation 10 or 20 minutes ? Did I get within 1.5m, or were they further away for all that time?

To be effective, contact tracing needs to occur as quickly as possible (at least within 4-5 days before infection develops, and ideally within a day or two). There are often built in delays - finding a case on Friday often means workplace contacts can't be traced until Monday.

It also needs to capture as many contacts as possible, as one missed contact may go on to infect others. The benchmark is generally that you need to find at least 80% of contacts. Part of the role of social distancing is to reduce the number of contacts that require follow up.

The app uses Bluetooth to exchange information between phones that are within 1.5m apart. Both phones have to have the app installed and Bluetooth turned on. They exchange reference codes, date and time, signal strength and duration of contact, which is stored on the phone.

They don't exchange locations or the actual name of the contacts. The app doesn't track your movements. The data are stored for 21 days. You can't access these data. You don't have to participate, and you can opt out at any time. You can disable it by turning off Bluetooth.

If someone with the app is diagnosed, they will be asked if the data can be uploaded to help with contact tracing. They don't have to submit the data on their phone if they don't want to. The data on the app cannot be used by law enforcement or other agencies.

This only complements, rather than replaces, the existing contact tracing process. All cases will still need to be interviewed. If only half the population have the app installed, then you would then expect roughly a quarter (0.5 x 0.5) of contacts to be detected by the app.

But having a list of contacts via the app may help confirm the level of contact, may find additional contacts, and would allow them to be contacted more quickly. It's not going to be perfect, but neither is human recall.

Some people will have privacy and security concerns, and this will be an important discussion to have. Ultimately, what this is trying to do is to make contact tracing more effective and efficient.

I've installed it. It's your choice about whether you want to.
Apple: apps.apple.com/au/app/covidsa…
Android: play.google.com/store/apps/det…