Risk Level Calculation for Contact Tracing: an Example of Apple IOS framework

You know in Australia there is a ‘Covidsafe app‘  for everyone.

covidsafe-app_1 The COVIDSafe app speeds up contacting people exposed to coronavirus (COVID-19). This helps us support and protect you, your friends and family. Please read the content on this page before downloading.
At the end of the Australian COVID-19 pandemic, users will be prompted to delete the COVIDSafe app from their phone. This will delete all app information on a person’s phone. The information contained in the information storage system will also be destroyed at the end of the pandemic. 

Here is the introduction video:

So, all those descriptions are trying to tell your information is safe and your privacy is protected with this app. By the way, the COVIDSafe app is the only contact tracing app approved by the Australian Government. I think this means it is the first official one.

This post is for viewers who want to understand a little bit deeper technical details about the technology used in this app. I will quote the document from Apple and keep it as simple as possible. I am not an IOS developer. I am just as curious as you, trying to understand how it measures the risk. And I am not sure if the COVIDSafe app used apple’s framework, LOL~

My only sources are from the webpages of the Australian Government Department of Health and Apple [iOS Framework Document] Exposure Notification April 2020. You can click these Keywords to learn more background knowledge around this app: COVIDSafe, Mesh Network; GDPR; DP3T; Beacon.

So, according to Apple’s document, the following diagram illustrates the general format of Exposure Risk Level Calculation:

Example Contact Tracing Apple IOS 03

Exposure Risk Level Parameters

  • Transmission Risk — An app-defined flexible value to tag a specific positive key. This value could be tagged based on symptoms, level of diagnosis verification, or other determination from the app or health authority.
  • Duration (measured by API) — Cumulative duration of the exposure. Days (measured by API) – Days since the exposure incident.
  • Attenuation (measured by API) – Minimum Bluetooth signal strength attenuation (Transmission Power subtract RSSI).
  • Level Value: The value, ranging from 1 to 8, that the app assigns to each Level in each of the Exposure Risk Level Parameters.
  • Level: The eight levels contained within each Exposure Risk Level Parameter.

Exposure Risk Level Parameter Weights (A, B, C, D)

  • The weights defined by the app (ranging from 0-100) that assign the relative importance to each of the Exposure Risk Level Parameters.


The following figure shows an example of an Exposure Risk Level calculated for a person named Alice who was exposed to a person (Bob) who was diagnosed positive. So, Alice is considered a primary exposed user.  With all the detected information from Alice’s phone, the ERL of Alice is estimated to be 6.

Example Contact Tracing Apple IOS 01

The fomular can be summarised as:

Exposure_Risk_Level = (TRP * 75 + DRP * 50 + DRP * 50 + ARP * 50) / 200, 


  • TRP = Transmission Risk Parameter ,
  • DRP = Duration Risk Parameter,
  • DRP = Days Risk Parameter,
  • ARP = Attenuation Risk Parameter,

are estimated by the Health Authority and stored in the configuration tables.


The following diagram shows how Exposure Risk Levels can be transferred along with the Diagnosis Keys via the key-sharing schema.

In this way, a secondary exposed user (Charles) who was recently near Alice, may optionally be notified by the app (should it determine that Alice’s exposure risk level is high enough that she is likely COVID-19 positive).Example Contact Tracing Apple IOS 02

Related news and Apple contact tracing pdf are in the links below:

As you can see, even this is a very good design, but it still suffers from two issues:

  1. People need to install this app. Ideally, if everyone installs the app, it will be much easier for State and territory health officials to tracing people and the source to prevent the virus from spreading.
  2. People need to ensure the COVIDSafe app is running and keep the Bluetooth on.

Anyway, for the general public health and human’s goodwill, please install the app if you were in Australia. Let us stand together and getting by this COVID19 pandemic safely.

Encourage the Author to create more useful and interesting articles.

All the money will be donated to the Standford Rural Area Education Program (reap.fsi.stanford.edu).



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