Technical Notes

Threshold Calculation

Epidemic thresholds: Epidemic thresholds were derived from historical Spokane County emergency department disease data using the Moving Epidemic Method (MEM), which is designed to detect seasonal and temporal patterns in disease incidence.

Emergency department (ED) visits with diagnostic codes for respiratory illness for flu seasons 2017-2018, 2018-2019, 2019-2020, and 2022-2023 were used for the calculations. Data for the 2021-2022 season were excluded because of atypically high COVID-19 transmission. RSV and influenza data from 2020-2021 were excluded due to atypically low influenza and RSV transmission. The unusually low incidence of flu and RSV during this season was likely influenced by universal masking and other community-wide disease control measures. Multiple waves of COVID-19 were accounted for by using the Moving Epidemic Method Web Application.

For the purposes of this dashboard and threshold calculations, respiratory illness season begins at week 40, typically the first week of October, and continues until week 39 of the following year.

Over epidemic threshold: Transmission of this illness is greater than what is typically observed throughout the year.

Below epidemic threshold: Transmission of this illness is less than or typical of what is observed throughout the year.

Trend Classification

The trend communicates whether the percentage of daily ED visits related to a respiratory illness are significantly increasing, decreasing or remaining stable. If there is a significant increase or decrease in the trend for a particular disease on any day of the previous week, it will be shown on this dashboard.

To determine the trend, a rolling binomial model was fit to daily ED visit data from the Centers for Disease Control and Prevention's Electronic Surveillance System for the Early Notification of Community-Based Epidemics (ESSENCE).

Data Sources

Spokane County ED visit data are gathered from ESSENCE.

Influenza hospitalization data are reported by local hospitals, and outbreak data are reported by long-term care facilities.

Influenza and COVID-19 mortality and outbreak data, as well as COVID-19 hospitalization data, are sourced from the Washington Disease Reporting System (WDRS). Because respiratory syncytial virus (RSV) is not currently a reportable condition, the dashboard does not include RSV hospitalizations, deaths or outbreaks.

The wastewater respiratory disease surveillance data for Spokane County is sourced from the Washington State Department of Health, in partnership with local water reclamation facilities. These data are normalized based on the size of the population served and number of viral gene particles detected.

Hospitalizations, Mortality and Outbreaks

Respiratory illness transmission is likely much higher than what is depicted in these visualizations. The visualizations represent more severe cases of COVID-19, influenza or RSV that result in ED visits, hospitalizations or deaths. It is very difficult to know exactly how many people in Spokane County have been infected to date since most people experience mild illness and may not seek a provider for testing.

COVID-19 hospitalization: Any inpatient hospital visit where COVID-19 is included in the diagnosis.

COVID-19 death: Any death in which COVID-19 or similar terms are mentioned on the death certificate, whether as a primary cause or contributing factor, is considered in the count.

Long-term care facility COVID-19 outbreak: As of Jan. 1, 2024, a COVID-19 outbreak in a long-term care facility (LTCF) is defined as

  • ≥2 epidemiologically linked cases of COVID-19 among residents within 7 days1, or
  • ≥2 epidemiologically linked COVID-19 cases among Health Care Personnel (HCP), AND
  • ≥1 case of COVID-19 among residents, AND
  • no other more likely sources of exposure for at least 1 of the cases.

This is a change from the previous, more sensitive, COVID-19 outbreak definition of one case in a resident or two epidemiologically linked staff cases over a 14-day period.

Influenza hospitalization: Any inpatient hospital visit where influenza is included in the diagnosis.

Influenza death: A laboratory-confirmed influenza-associated death is defined as a death resulting directly or indirectly from a clinically compatible illness that was confirmed to be influenza by an appropriate laboratory test. There should be no period of complete recovery between the illness and death. Laboratory criteria for diagnosis include:

  • Influenza virus isolation in tissue cell culture from respiratory specimens
  • Reverse-transcriptase polymerase chain reaction (RT-PCR) testing of respiratory specimens
  • Immunofluorescent antibody staining (direct or indirect) of respiratory specimens
  • Rapid influenza diagnostic testing of respiratory specimens
  • Immunohistochemical (IHC) staining for influenza viral antigens in respiratory tract tissue from autopsy specimens
  • Four-fold rise in influenza hemagglutination inhibition (HI) antibody titer in paired acute and convalescent sera

Long-term care facility influenza outbreak: An influenza outbreak is defined as: a sudden increase in acute febrile respiratory illness over the normal background rate in an institutional setting OR when any resident of a long-term care facility (LTCF) tests positive for influenza.

1 Washington State Department of Health. Interim COVID-19 Outbreak Definition for Healthcare Settings. January 25, 2024.