The two companies were set to release application programming interfaces (APIs) to assist developers in creating apps with interoperability between Android and iOS devices in May 2020. Then the two “will work to enable a broader Bluetooth-based contact tracing platform.” At both stages, the companies promise, “We will openly publish information about our work for others to analyze.”


The attempt to keep up with the spread of a contagious disease by isolating cases and employing contact tracing is an established strategy, and it’s in use with COVID-19. A study by the Centre for the Mathematical Modelling of Infectious Diseases of the London School of Hygiene & Tropical Medicine offered a dramatic estimate of the potential for tracking in a March 5, 2020, study titled “Feasibility of controlling COVID-19 outbreaks by isolation of cases and contacts.”

Using a stochastic transmission model, the researchers concluded, “In most scenarios, highly effective contact tracing and case isolation is enough to control a new outbreak of COVID-19 within 3 months.” With few exceptions, the current manual methods for tracing contacts are intensive and slow. Interviews and phone-call follow-ups can’t keep up with the speed of the infection.

What’s needed is more effective tracing of everyone’s close contacts and notifications from a widely installed system—in short, an automated tracking system that doesn’t require effort or management from users, one that guarantees privacy but can open immediate lines of communication from public health officials to warn users of their possible contact with a person who has tested positive for the disease. The system chosen by the Google-Apple collaboration, and many others, is Bluetooth and Bluetooth LE.


Most digital citizens worldwide already carry a very efficient tracking device with them. There are literally billions of smartphones that move around all day within close vicinity of their owners. Built into both the Apple and Android versions of smartphones are two active Bluetooth systems. Normal Bluetooth lets you connect your phone to other computer systems or peripherals like speakers or even your smart watch. The other, Bluetooth LE (low energy), automatically generates and broadcasts a stream of “chirps” on a mostly continuous basis.

MIT’s own COVID tracking research, called PACT (Private Automated Contact Tracing), also depends on Bluetooth, and, according to a white paper from the initiative, “These chirps are not linkable to the device (smartphone) or its owner, and are changed (‘rotated’) frequently so they cannot be used to track the device. Each device keeps track of the chirps it transmits, as well as of the chirps it receives.”

A key element of this silent contact is the recording of your proximity to the other phones in the area. “The distance between the two devices…is used as a proxy for the distance between the two individuals, and it is an important factor in determining how likely one of them is to infect the other, should they happen to become infectious. Another factor in determining this likelihood is the duration of contact, which our [MIT’s FACT] protocol also tries to provide an estimate for.”

Your phone, then, is constantly chirping out its presence to others in the area, while also receiving and recording the locations of other phones also chirping in the location. You might wonder why phones do this. Turns out there are practical uses; for instance, the Apple Find My iPhone app or for advertisers sending those curiously relevant ads that pop up on your phone when you enter some retail establishments. Your phone announced (chirped) your presence when you came in the door.

Your phone is an ideal tracker because with onboard GPS, it can always know where you are, and it’s constantly sending out its location while recording its proximity to other phones in the vicinity. If a person is identified as positive for the virus, the chirp values his or her phone has broadcast in the past three weeks could be publicly broadcast and others could check to see if they might have been exposed. If so, they could contact health authorities for advice on a course of action. We can probably expect to hear much more about Bluetooth-based tracing systems and the relevant privacy issues soon.

See the full-text PDF of the PACT protocol for MIT’s COVID-19 tracing system at

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