For basic information on the disease and how individuals can mitigate the spread, the following resources provide excellent information:

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  • What is the definition of a "close contact?"


    Both When to Test Calculators are based on CDC guidelines. Current guidelines define a “close contact” as someone who was within six feet of an infected person for a total of at least 15 minutes over the course of 24 hours. One exception to that rule is applicable only to schools: Students who were between three and six feet of an infected person are not considered close contacts as long as they were in school and “correctly and consistently” wearing masks.

    CDC — Close contact definition 

  • What does Emergency Use Authorization (EUA) mean?


    In the US, the use of medical tests is regulated by the Food and Drug Administration (FDA). These tests can only be used in this country once they’ve undergone a long approval process. In early 2020, the US was in desperate need of tests for COVID-19; we needed them as soon as possible. For this reason, the US Secretary of Health and Human Services declared that the FDA could use Emergency Use Authorization to make COVID-19 tests available more quickly.

    As its name suggests, EUA is a seldom-used type of authorization which is only allowed during public-health emergencies. It enables the FDA to authorize the use of medical products (including tests) that haven’t undergone the long approval process, provided that “certain criteria are met.” At a minimum, the product must have known and potential benefits that outweigh its known potential risks. In addition, there must be “no adequate, approved, and available alternatives” to the product receiving the EUA.


    There are currently no COVID-19 tests that have complete approval from the FDA. Those that have EUA have undergone a shortened version of the approval process, which ensures that they meet the criteria mentioned above, among other requirements. The When to Test Calculator for Organizations assumes that your workplace or school is using a test that has received EUA.

    FDA — Emergency Use Authorization

  • What kinds of tests can be used for COVID-19 screening or diagnostic testing?


    Both kinds of testing can be done using either a PCR (also known as molecular) or antigen test.

    PCR tests look for pieces of the virus’s genetic material. They are the most accurate kind of test for COVID-19 currently available.

    Antigen tests look for viral proteins. They are slightly less accurate than PCR tests.

    Antibody tests (also known as serology tests) look for antibodies to SARS-CoV-2 (the virus that causes COVID-19). Those antibodies can come from a past COVID-19 infection or from a COVID-19 vaccine. This kind of test cannot be used to diagnose an active COVID-19 infection.

    CDC — Interim Guidance for Rapid Antigen Testing

  • What is the difference between point of care (POC) testing and lab-based testing?


    In point-of-care (POC) testing, all testing steps, including sampling and analysis, take place close to or near the patient. For the When to Test Calculators, this means that testing is completed either on-site at your organization’s location or in a health-care provider's office, at a testing site, or in your home. Point-of-care tests give you results quickly. Most of them are antigen tests, but some point-of-care PCR tests are available, as well.

    In lab-based or off-site testing, samples are gathered on-site at your organization or other testing location and are sent to a central lab for processing and analysis. Lab-based tests are generally PCR tests, which are highly accurate. However, they take longer to return results than point-of-care tests do. In some cases, the turnaround time of lab-based tests is not fast enough to prevent an outbreak in an organization.

    CDC — Guidance for COVID-19 Point-of-Care Testing

  • What is R0 (R-naught)?


    R0 (R-naught) is a measure of the average number of people who will contract a contagious disease from one infected person. It’s a measure of viral transmissibility, or how fast the disease is spreading.

    When R0 is exactly 1.0, it means that, on average, one infected person will infect one other person. When R0 is less than 1.0, it means that the spread of the disease is decreasing within the community. When R0is above 1.0, the spread of disease is increasing. The more transmissible a disease or disease variant is, the higher its R0.

    The When to Test Calculator for Organizations bases its default R0 on CDC estimates regarding the transmissibility of the most prevalent variant of COVID-19 in the US, which is currently the Delta variant. The Calculator's default R0 for Typical conditions reflects the CDC's low-end estimate of Delta's R0, while the default setting for Hotspot conditions reflects a higher-end estimate of Delta's R0. If you know which variant is most common in your area and wish to change the R0 in your scenarios to reflect that variant, click on "Show Advanced Settings," then "Main Calculator Settings,” and scroll down to "Estimated R0."

    CDC — US COVID-19 Cases Caused by Variants

    CDC — What You Need to Know about Variants

    World Health Organization — Tracking SARS-CoV-2 Variants

    Johns Hopkins Medicine — New Variants of Coronavirus


  • How do COVID-19 variants affect testing?


    The CDC and FDA, in partnership with the major testing laboratories, continue to carefully monitor the emergence of variants and how their genetic characteristics might affect testing.

    NOTE: As part of its calculations, the When to Test Calculator for Organizations includes a default measurement of how transmissible the virus is. That measurement is called R0 (“R-naught”) or the basic reproduction number for the virus. follows CDC guidance to set the Calculator’s default for R0. If you would like to set a specific R0 in your scenarios, you can do so under Advanced Settings: Main Calculator Settings.

    CDC — What You Need to Know About Variants

    CDC COVID — 19 Pandemic Planning Scenarios

    FDA Issues Alert Regarding SARS-CoV-2 Viral Mutation

  • What is prevalence?


    For the purposes of the When To Test Calculators, prevalence is the percentage of individuals in a population who are infected with COVID-19 at any given moment. If there are 100 people in a building and one person has COVID-19, then the prevalence of COVID-19 in that building is 1%.

    The prevalence default settings for the When To Test Calculator for Organizations are 1% for typical conditions and 3% for hotspot conditions. To adjust these settings, click on the Show Advanced Settings button on the results page, open the Main Calculator Settings tab, and scroll down to the Prevalence settings (green arrows below).

    The When To Test Calculator for Individuals uses prevalence information that is estimated at the county level.