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




  • 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 

  • I've already had COVID-19. How will that affect my testing recommendation from the When to Test Calculator for Individuals?



    If you've been infected with SARS-CoV-2 (the virus that causes COVID-19), your risk of getting infected again is lower for a period of time. However, it's not clear exactly how much lower that risk is; it might depend on how severe your case was and what variant you were infected with. It's also not clear how long the protection lasts.

    For those reasons, the When To Test Calculator for Individuals assumes that prior infection only gives you a relatively small amount of protection from future disease, and that the protection is temporary.

    CDC — Reinfection with COVID-19

  • The When to Test Calculator for Individuals recommended that I get tested for COVID-19. What should I do now?



    The When To Test Calculator for Individuals is a tool intended to help you decide whether you should get tested for COVID-19 in order to keep from spreading the disease to other people. If you get this recommendation, your risk of spreading the disease will be lowest if you get tested before coming into close contact with anyone outside your household. (This Calculator is a decision-support tool only. Results are provided for informational purposes and should not be construed as medical advice.)

    COVID-19 tests are available at no cost to anyone in the nation, including the uninsured. Visit the US Department of Health and Human Services website to find a free testing site near you. You can also contact your health-care provider or your state or local public health department for more information.

  • What's the difference between PCR and antigen tests?


    PCR tests, also known as molecular tests, look for pieces of the virus’s genetic material using a lab technique called polymerase chain reaction (PCR). They are the most accurate kind of test for COVID-19 currently available.

    PCR samples are usually sent for processing and analysis in a lab, and results can take anywhere from a few hours to several days to arrive. Lab-based PCR tests tend to be more costly than other types of test. Some point-of-care PCR tests are now available, but they typically require purchase of a dedicated instrument and training to run the tests. If you want to use a point-of-care PCR test to screen members of an organization, you may need to purchase multiple instruments and train several staff members for this purpose. Point-of-care PCR tests typically return results in under an hour.

    Antigen tests look for viral proteins. These tests typically provide results within minutes and are very good at accurately identifying people who are shedding a lot of virus particles. They are not as good as PCR tests are at finding people who aren't carrying as much virus, including children and asymptomatic adults. If you are using this type of test in an organizational setting, be sure to understand how well the particular brand you use works in the population you’ll be testing (adults vs. children, people with symptoms vs. people without symptoms). Many antigen tests still require trained staff to run them, but a few are now available over the counter. Antigen tests tend to be the least expensive type of test.

    Because of their decreased sensitivity, antigen tests have the following limitations:

    • If you are using antigen tests to screen members of an organization, you may need to test more frequently than you would if you were using a PCR test in order to prevent an outbreak.
    • Positive results on antigen tests whose specificity is >99% are usually correct, but negative results may need to be confirmed with a PCR test.

    Remember, no test is perfect. 

    Coronavirus Disease 2019 Testing Basics

    CDC - Interim Guidance for Rapid Antigen 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


  • 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).