New technical proposals to track, contain, and fight COVID-19 are coming out nearly every day, and the distinction between public health strategies, technical approaches, and other terms can be confusing. On this page we attempt to define and disambiguate some of the most commonly used terms. Bookmark this glossary—we intend to update it with new terms and definitions regularly.

For more information on COVID-19 and protecting your rights, as well as general information on technology, surveillance, and the pandemic, visit our collection of COVID-19-related writing.

Contact tracing: This is the long-standing public health process of identifying who an infected person may have come into contact with while they were contagious. In traditional or manual contact tracing, healthcare workers interview an infected individual to learn about their movements and people with whom they have been in close contact. Healthcare workers then reach out to the infected person’s potential contacts, and may offer them help, or ask them to self-isolate and get a test, treatment, or vaccination if available.

Digital contact tracing: Some companies, governments, and others are experimenting with using smartphone apps to complement public health workers’ contact tracing efforts. Most implementations focus on exposure notification: notifying a user that they have been near another user who’s been diagnosed positive, and getting them in contact with public health authorities. Additionally, these kinds of apps—which tend to use either location tracking or proximity tracking—can only be effective in assisting the fight against COVID-19 if there is also widespread testing and interview-based contact tracing. Even then, they might not help much. Among other concerns, any app-based or smartphone-based solution will systematically miss groups least likely to have a smartphone and most at risk of COVID-19: in the United States, that includes elderly people, low-income households, and rural communities.

Contact tracing using location tracking: Some apps propose to determine which pairs of people have been in contact with each other by collecting location data (including GPS data) for all app users, and looking for individuals who were in the same place at the same time. But location tracking is not well-suited to contact tracing of COVID-19 cases. Data from a mobile phone’s GPS or from cell towers is simply not accurate enough to indicate whether two people came into close physical contact (i.e. within 6 feet). But it is accurate enough to expose sensitive, individually identifiable information about a person’s home, workplace, and routines.

Contact tracing using proximity tracking: Proximity tracking apps use Bluetooth Low Energy (BLE) to determine whether two smartphones are close enough for their users to transmit the virus. BLE measures proximity, not location, and thus is better suited to contact tracing of COVID-19 cases than GPS or cell site location information. When two users of the app come near each other, both apps estimate their proximity  using Bluetooth signal strength. If the apps estimate that they are less than approximately six feet apart for a sufficient period of time, the apps exchange identifiers. Each app logs an encounter with the other’s identifier. When a user of the app learns that they are infected with COVID-19, other users can be notified of their own infection risk. Many different kinds of proximity tracking apps have been built and proposed. For example, Apple and Google have announced plans for an API to allow developers to build this kind of app.