WASHINGTON — The latest statewide modeling report shows COVID-19 transmission trending downwards in western Washington and upwards in eastern Washington. The report estimates differences between counties by combining modeling estimates of how the disease is spreading with data on reported cases.

The report estimates the effective reproductive number – the measure of how many new infections a single COVID-19 case will produce – for different parts of the state. A reproductive number below one means the number of new cases are declining.

The report is based on data from May 3-12. Findings are specific to that time period and include:

  • The reproductive number varied in different parts of the state.
  • The report estimates the average reproductive number was below one in western Washington and above one in eastern Washington.
  • Excluding Yakima County, the estimated reproductive number for eastern Washington was fractionally above one.
  • The majority of cases were reported in King and Yakima counties.
  • New case counts were trending downwards in King County and were steadily increasing in Yakima County.
  • Yakima, Douglas, and Chelan counties had the highest number of cases relative to their population.

County rates will have changed in the most recent data, including data under review for county variance applications.

The geographical differences seen in the report likely have multiple causes, including differences in testing. It’s also likely there are differences in infection rates due to behavioral and socioeconomic factors that affect COVID-19 exposure risk.

“As we continue to see differences from county to county, our efforts to support different regional needs for reopening become increasingly critical—and so does our request that people avoid traveling outside county lines to areas with fewer cases,” said Secretary of Health John Wiesman. “I’m asking everyone to continue their hard work to protect others in our community and our state by following public health guidance.”

The Department of Health (DOH) worked with Bellevue-based Institute for Disease Modeling (IDM) and the Microsoft AI for Health program to develop the report, which updates previous statewide modeling reports with more recent data.

The report is one of many data sources the state is using to assess COVID-19 risk levels. These data sources help inform the state’s Safe Start planning and guidance.

For more information on COVID-19, visit the Department of Health’s website or call 1-800-525-0127. You can also text the word “coronavirus” to 211-211 to receive information and updates.