Community Transmission Index

The community transmission index can help you better understand how widely COVID-19 is spreading in your area. When you leave your home and go into your community, a lower rate means you have a lower risk of exposure to the virus that causes COVID-19. A high rate means there is significant evidence of community spread and you should take precautions against infection.

How is the community transmission index calculated?

The community transmission index reflects the number of newly confirmed cases in the last 14 days per 100,000 residents. Using the population size in the calculation helps us more easily compare larger and smaller counties. A larger county would be expected to have more cases because of the larger population, but expressing the rate per 100,000 residents enables a more equal analysis.

How should I interpret the numbers?

Transmission levels are grouped as follows:

  • High: greater than 100 cases per 100,000 county residents
  • Moderately High: 51-100 cases per 100,000 county residents
  • Moderate: 11-50 cases per 100,000 county residents
  • Low: 0-10 cases per 100,000 county residents

What else do I need to know about these data?

Due to variations in the availability of testing, test-seeking behavior, local outbreaks, and widespread testing in congregate living settings, these data may not accurately represent true community transmission and should be considered with additional factors affecting the community in consultation with District Public Health, who can provide more guidance about the scenario in your county and if your transmission category is affected by outbreaks in specific settings. In addition, counties may change categories weekly, particularly smaller counties where a change of just a few cases could lead to a fairly substantial change in the mathematical calculation.

Please consult additional resources to understand trends and other factors affecting your county.


The following graphs show the 14-day community transmission index for each county in the Coastal Health District since mid-June. These graphs will be updated weekly.

See also: Daily Average of New Cases | Daily Case Rate per 100K | Hospitalization Data | Percentage of Positive Tests | Cumulative Cases | Cases by Zip Code | Cases and Deaths by Age | Weekly Testing Numbers

Data updated: Sept. 21, 2020