COVID-19 spreads easily between humans. The CDC tracks how much COVID-19 spreads, as well as how likely people are to be exposed to it with a measurement known as “the level of community transfer.”† You can use COVID-19 County Check Tool for a snapshot of your county’s level of community transmission over the last 7 days. The tool also shows masking instructions based on how the virus spreads in your county.
How the CDC measures the county level of community transmission
The CDC looks at two numbers – totally new cases and percent positivity – to determine the level of societal transfer.
- Total new cases refers to a county’s frequency of new COVID-19 infections, reported over the last 7 days, per. 100,000 inhabitants. To calculate this number, the CDC divides the total number of new infections by the total population of that county. The CDC multiplies this number by 100,000.
- Percent positivity refers to the percentage of positive COVID-19 tests in a county over the past 7 days. This figure is based on reports from states about a specific type of test known as a nucleic acid amplification test (NAAT). To calculate this number, the CDC divides the number of positive tests by the total number of NAATs performed in that county. The CDC multiplies this number by 100 to calculate the percentage of all tests that were positive. Learn more at Calculation of SARS-CoV-2 laboratory test percentage positivity.
A higher number of overall new cases and a higher percentage of positivity correspond to a higher level of community transfer, as shown below. If the values for each of these two metrics are different (for example, if one indicates moderate and the other low), the higher of the two should be used to make mask use decisions in a county.
When data is unavailable for one metric, the other is used to determine the level of Community transmission. When data for both metrics are not available, the county level of community transmission cannot be determined.
|Community transmission levels||Low transmission||Moderate transmission||Significant transmission||High transmission|
|Total number of new cases per 100,000 people in the last 7 days||0-9.99||10-49.99||50-99.99||≥100|
|Percentage of NAATs that are positive in the last 7 days||0-4.99%||5-7.99%||8-9.99%||≥10.0%|
County level for community transfer and masking
People and local decision makers should consider the county level of community transfer when making masking decisions. Although COVID-19 vaccines approved in the United States are extremely effective in limiting the spread of COVID-19 and preventing serious disease, vaccination in some parts of the country remains low. Layered prevention strategies – such as masking along with being vaccinated – can help reduce the spread of COVID-19 further. CDC’s updated guide, issued in July 2021, recommends using county community transmission levels over the last 7 days to help determine who to mask and under what circumstances. See below for a quick reference on when to mask:
|County level of Community transmission||Guidance|
|High or significant transmission||Everyone should wear a mask in a public, indoor setting.|
|Moderate or low transmission||Unvaccinated persons should wear a mask in a public, indoor environment.|
The requirements for the mask vary from place to place. Make sure you follow local laws, regulations, rules or guidelines.
Visit COVID Data Tracker* to learn more about the level of community transfer in your amt. Please note that county-level data is not available for territories. Visit the relevant one to get the most accurate and up-to-date data for any county or state the Danish Health and Medicines Authority’s website.
*COVID Data Tracker can display data that differs from state and local websites. This may be due to differences in how data was collected, how metrics were calculated, or the timing of web updates.
†COVID Data Tracker shows community transmission at two levels, county and state / jurisdiction level. For both, the CDC uses aggregate data from the state / jurisdiction to determine the societal transmission indicator. There are different reporting methods for aggregate data at the county level and aggregate data at the state / jurisdiction level. That indicator of societal transmission at state level provides an “at-a-glance” picture of a state’s overall level of societal transmission. That Community transmission level at county level provides an image at the local level and is used as a basis for CDC’s updated guide.