1. COVID-19 TESTING
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.
Variants are mutated forms of SARS-CoV-2, the virus that causes the disease COVID-19. The variants that cause concern are ones whose mutations make them more likely to spread from person to person, cause more severe disease, and/or are able to more easily infect people who have been vaccinated.
All COVID-19 tests that have Emergency Use Authorization (EUA) from the FDA must be checked on an ongoing basis to make sure that they are able to detect different variants of the virus. If a test has any difficulty detecting a variant, the FDA must be notified immediately.
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. WhentoTest.org 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.
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.
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.
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.
Isolation and quarantine mean essentially the same thing and serve the same purpose: They help protect the public by preventing exposure to people who have or may have a contagious disease.
Isolation separates people who are infected from those who aren’t.
Quarantine separates people who were exposed to a contagious disease from others and restricts their movements while they wait to see if they are infected.
The When to Test Calculator for Individuals recommended that I get tested for COVID-19. What should I do now?
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. In addition, you can purchase over-the-counter at-home COVID-19 tests at many pharmacies and other retail oulets, as well as online.
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.)
Prevalence is the percentage of people in a population who have a disease. So if there are 100 people in your area and 10 of them have COVID-19, the prevalence of COVID-19 in your area is 10%.
Technically, the only way to determine the true prevalence of COVID-19 in a population is to test every single person in the population to see who has it and who doesn't. Since that's not feasible, the Calculators use a sophisticated machine-learning model that looks at all of the available COVID-19 data for a particular state and county and develops a "predicted prevalence" for that location.
For US territories and areas outside the US, the Calculators use a prevalence of 1% for typical conditions, and 3% for hotspot conditions.
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.
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.
What’s the difference between COVID-19 screening (aka asymptomatic screening) and diagnostic testing?
Screening for COVID-19, also referred to as screening testing or asymptomatic screening, means testing your population to identify infected people who are not showing symptoms. Diagnostic testing for COVID-19 is performed when infection is suspected, such as when the person:
- Has COVID-19 symptoms or
- Has no symptoms, but has had a recent known or suspected exposure to SARS-CoV-2 (the virus that causes COVID-19).
Asymptomatic screening is typically a regularly scheduled event (e.g., 1x/week). Along with other COVID-19 mitigation strategies, screening can help prevent outbreaks in your school or workplace.
Not many tests have authorization from the FDA specifically for screening testing. However, the FDA supports the use of COVID-19 tests that have Emergency Use Authorization (EUA) when used “off-label” under the supervision of a physician or other prescriber.
NOTE: The When to Test Calculators assumes that tests that do not have FDA authorization for screening are used off-label under practitioner guidance.
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.
Antibodies are proteins created by your immune system to help you fight off infectious microbes. Your body makes them after you’ve been infected or vaccinated.
Antibody tests (also known as serology tests) look for antibodies to SARS-CoV-2 (the virus that causes COVID-19). They cannot be used to diagnose an active COVID-19 infection, because it can take up to three weeks after the infection for your immune system to make antibodies. These tests can only be used to determine whether someone had COVID-19 or a COVID-19 vaccine in the past.
Sensitivity refers to how well a test designates someone who HAS the disease as positive.
If you have a test with 95% sensitivity and you test 100 people who have COVID-19, 95 will test positive (true positive) and 5 will test negative (false negative). Tests with high sensitivity will find most cases of the virus. They will return few false negatives.
Specificity is how well a test designates someone who DOESN’T have the disease as negative.
If you have a test with 95% specificity and you test 100 people without COVID-19, 95 will test negative (true negative) and 5 will test positive (false positive). People who test positive using tests with high specificity are very likely to truly have the disease. These tests return few false positives.
In an ideal world, all tests would be both highly sensitive and highly specific. Unfortunately, that’s not possible most of the time. It’s also important to acknowledge that no test is perfect - no test will have 100% sensitivity and specificity.
2. COVID-19 MITIGATION
Yes. The Calculator uses an estimate of how well the vaccines available in the US protect against the variant that is currently dominant in the country. This estimate is based on the performance of all available vaccines, not on any one vaccine brand or type.
If you are using the Calculator for Organizations and want to adjust the Calculator's default so that it matches a specific vaccine, go to the Advanced Settings, click on "Main Calculator Settings," and look for "Vaccine Effectiveness."
CDC — COVID-19 Vaccine Effectiveness Research
I'm vaccinated. How will that affect my testing recommendation from the When to Test Calculator for Individuals?
All currently authorized COVID-19 vaccines are highly effective at preventing hospitalization, severe disease, and death due to the SARS-CoV-2 virus. People who are fully vaccinated are also less likely to become infected with SARS-CoV-2 at all, which means they are less likely to spread COVID-19 than unvaccinated people are. For that reason, the number of contacts that would trigger a recommendation to get tested will be higher if you're fully vaccinated.
How does mask-wearing affect my test recommendation from the When to Test Calculator for Individuals?
The When To Test Calculator for Individuals tells you whether you need to be tested in order to prevent the spread of COVID-19 in your community.
Wearing a mask significantly reduces both your risk of getting COVID-19 and the chance that you'll pass on the disease if you do get infected. As a result, the number of contacts that would trigger a recommendation to get tested will be higher if you wear a properly fitted mask when you're in close contact with other people.
3. USING THE WTT CALCULATOR FOR INDIVIDUALS
I am/my organization is in a US territory or outside of the US. How do I know if I'm in a COVID-19 hotspot?
You are in a COVID-19 hotspot if:
- There have been at least 100 cases of COVID-19 per 100,000 people in your area over the past seven days, and/or
- The test positivity rate in your area is currently 10% or higher.
The number of cases per 100,000 people is called the incidence rate. If you are outside the United States, you can find your country's seven-day incidence rate here.
- American Samoa: You are in a hotspot if your territorial risk level is red.
- Guam: Check test positivity rate on your territory's dashboard.
- Northern Mariana Islands: Your territory does not report test positivity rate or seven-day incidence. You can find the number of cases in the last seven days here.
- Puerto Rico and US Virgin Islands: Scroll down to the table labeled "Reported cases, deaths and other trends by county" and check the column labeled "Per 100,000."
The When To Test Calculator for Individuals uses two different parameters to tell you whether and when you need to get tested: 1) Your risk of having or getting infected with COVID-19, and 2) Your risk of spreading it to other people.
The Calculator figures out how high your risks are based on four inputs. Three of those inputs are protective measures that you can take against COVID-19:
- Distancing (The Calculator captures this information by asking about close contacts.)
The fourth input is the prevalence of COVID-19 in your area.
Prevalence is the percentage of people in a population who have a disease. So if there are 100 people in your area and 10 of them have COVID-19, the prevalence of COVID-19 in your area is 10%.
Technically, the only way to determine the true prevalence of COVID-19 in a population is to test every single person in the population to see who has it and who doesn't. Since that's not feasible, the Calculator uses a sophisticated machine-learning model that looks at all of the available COVID-19 data for a particular state and county and develops a "predicted prevalence" for that location.
The Calculator uses those "four pillars" - vaccination, masking, distancing, and predicted prevalence - and combines them with information about your behavior over the past seven days and what you expect your behavior to be over the next seven days. The result is a calculated estimate of the following:
- Your risk of getting COVID-19 over the past seven days
- Your risk of getting COVID-19 over the next seven days
- Your risk of spreading COVID-19 to others over the past seven days
- Your risk of spreading COVID-19 to others over the next seven days
Based on those four risk factors, the Calculator determines whether you should consider getting tested for COVID-19.
Note: If you are in a US territory or outside of the US, the Calculator asks if you are in a COVID-19 hotspot. If you are not in a hotspot, the Calculator assumes that the prevalence of COVID-19 in your location is 1%. If you are in a hotspot, it assumes that the prevalence in your area is 3%.
DataRobot - The company that developed the machine-learning model behind the When To Test Calculator for Individuals.
I am going to be traveling to a different geographic location next week. Which location should I use as my input in the When To Test Calculator for Individuals?
The When to Test Calculator for Individuals uses two different parameters to tell you whether and when you need to get tested: 1) Your risk of having or getting infected with COVID-19, and 2) Your risk of spreading it to other people. It uses the prevalence of COVID-19 in your current location to help it determine how likely it is that you will get infected. It uses the number of people you plan to see next week to help it determine how likely it is that you'll spread the disease.
Given that, there are a couple of different ways you can handle this situation. One option would be to start by using the location you're in right now. Then recheck your results with the Calculator next week, using the location you're in at that time. When you check the Calculator this week, you'll find out if you need to get tested now, before you travel to your next location. By checking the Calculator again next week, you can find out if you need to get tested once you're in your new location.
Another option would be to run the Calculator two times right now, to see how your results might differ in one place versus the other. Just remember that the prevalence of COVID-19 in either location might change over the course of the week. When in doubt, come back to the Calculator to find out if your recommendation has changed.
I checked my recommended testing strategy with both When To Test Calculators, and I got different results. Why did that happen?
The two Calculators have slightly different goals and base their recommendations on different inputs.
The goal of the When To Test Calculator for Individuals is to reduce the spread of disease in a given geographic area at a specific time. It does that by recommending that an individual get tested when it's more likely that they might be about to pass COVID-19 on to others. (This Calculator is a decision-support tool only. Results are provided for informational purposes and should not be construed as medical advice.)
The goal of the When To Test Calculator for Organizations is to decrease the overall risk of an outbreak within a workplace or school. It does that by evaluating the COVID-mitigation strategies in place at the organization and recommending the appropriate level of testing needed to complement those strategies.
For all of those reasons, when you use the two tools you might get different results. If your personal and organizational recommendations differ, consider taking both into account when deciding how often you should get tested for COVID-19.
How do my past and future contacts help the When To Test Calculator for Individuals figure out whether I need to be tested?
The Calculator tells you whether you need to be tested in order to prevent the spread of COVID-19 in your community.
The number of people with whom you were in contact last week - along with the prevalence of COVID-19 in your area - tells the Calculator how likely it is that you have been infected with COVID-19.
The number of people with whom you plan to be in contact next week tells the Calculator how likely it is that you will infect other people.
The Calculator uses those factors and other inputs to develop a recommendation about whether you need to be tested in order to help stop the spread of COVID-19 in your area.