Apple and Google plan to update their joint exposure notification project — which forms the backbone of several COVID-19 exposure alert apps like Canada’s own COVID Alert — with more sophisticated risk assessment capabilities.
In short, the changes should allow the exposure notification API to quantify ‘reliable’ and ‘unreliable’ signals and use new algorithms to reduce the number of false positives. It could also mean more people getting alerts about potential exposures to COVID-19.
To start, it’s important to understand how the exposure notification system operates now to put the changes in perspective. Currently, the API created by Apple and Google works by broadcasting random, anonymous and non-identifying codes over Bluetooth from your smartphone. Other nearby phones running the exposure notification system can pick up those codes and trade their own codes back.
These codes form an anonymous record of potential exposure to nearby people stored locally on your phone. While the codes can’t identify who you are, who you were near or where you were, they can include details like the strength of the Bluetooth connection and how long the devices were within range of each other. It’s these data points that currently impact the exposure risk of a contact you had with someone else. Further, these data points are pivotal in how the updated API works to improve accuracy.
Bluetooth signal strength helps apps determine risk of exposure
For those unfamiliar with the exposure notification system or apps like COVID Alert, when someone gets a positive COVID-19 test result, they also get a verification key they can enter into the app. This key allows positive users to upload the list of codes stored on their phone. Other phones with the COVID Alert app can check the list for matching codes — matches indicate potential exposure to someone with COVID-19. Further, COVID Alert uses data about the strength of the Bluetooth signal and the amount of time the phones were in range to determine how much of an exposure risk the person had.
Although a solid system for both alerting people to possible COVID-19 exposure while also protecting user privacy, it isn’t without flaw. Mainly, various factors can impact the strength of Bluetooth signals, such as the location of a device (in your hand versus in a backpack or purse), walls and other surroundings. Because of this, exposure notification apps often require a higher threshold of Bluetooth strength to label a potential exposure as ‘risky.’
Apple and Google released a technical paper detailing plans to change this part of the API to assess Bluetooth strength more accurately. In short, the exposure notification API will be able to check for rapidly fluctuating signal strength, which could indicate interference. For example, if Bluetooth signals bounce off a wall, it could make it harder to determine the distance between the source and the receiver. The API can mark this data as unreliable.
Apple and Google’s exposure notification API will be able to apply new algorithms to that fluctuating signal to better estimate how far away the Bluetooth signal source is. Ultimately, that should help reduce false positives where the API thinks the source is closer than it is, and prevent unnecessary exposure alerts. If you’re interested in reading the technical details, the paper is available here.
Initial reports indicate new algorithms improve the reliability of distance estimates
As an example, BBC News reported that Britain’s NHS team was so confident in the improved distance estimates that they were able to lower the app’s exposure notification threshold significantly. In other words, it will take less to trigger a notification, but it’s also more likely that the exposure notification users receive is correct and not a false positive.
While less false positives are certainly welcome, the change could eventually mean more exposure alerts for people. If the change is as effective as reported, then the developers who make apps like COVID Alert can tweak the thresholds to trigger an exposure alert. These parameters are ultimately up to the developers and public health agencies in each country, so it remains unclear if COVID Alert will benefit from the change, or how much it would impact Canadians.