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Public Health - Seattle & King County

Influenza surveillance

Annual influenza epidemics are a significant cause of morbidity, mortality, and health care costs. Those at highest risk for influenza-related hospitalization and death include infants, the elderly, and persons with chronic underlying medical conditions and obesity. Annual influenza vaccination reduces the risk for illness and is recommended for all persons 6 months of age and older. Influenza is spread primarily by respiratory droplets (droplet spread) produced when an infected person coughs or sneezes.

Purpose of surveillance

  • To detect the emergence of novel influenza
  • To monitor influenza activity in the community
  • To identify clusters of severe illness and outbreaks of influenza in institutional settings
  • To monitor mortality due to laboratory-confirmed influenza

Public Health - Seattle & King County conducts influenza surveillance in order to provide information on local influenza activity to health care providers and the public. Because individual cases of influenza are not reportable unless they are fatal, Public Health monitors data from several sources in order to understand when influenza has arrived, is most active, and subsides in our community. The main sources of influenza surveillance data are:

  • King County Sentinel Influenza Providers - Volunteer physicians obtain specimens of respiratory secretions from a subset of patients who present with influenza-like illness (ILI) and submit them for viral culture at the Public Health Laboratory. This system helps us confirm when influenza has arrived in our area. The Public Health Laboratory, a National Respiratory and Enteric Viral Surveillance System Collaborating Laboratory, determines the type and subtype of influenza (A or B) and submits a sample of influenza viruses to the Centers for Disease Control and Prevention (CDC) for further antigenic characterization. Specimens submitted to the Public Health laboratory are also tested for other common respiratory viruses, including parainfluenza virus, respiratory syncytial virus (RSV) and enterovirus.

  • Rapid Antigen Test Submissions and Results - Several local microbiology laboratories report influenza rapid antigen test results each week. When influenza is circulating, the number of rapid antigen tests ordered, and proportion positive can reflect the level of influenza in the community. Rapid tests are generally reliable when positive, but when influenza activity is low, the rate of false positive results is increased.

  • Hospital Emergency Department Visits for Influenza-Like Illness ( ILI ) - Emergency department chief complaint data from 19 of the county's 20 hospitals are monitored for visits due to influenza-like illness (fever, sore throat, cough, headache, myalgias, etc). This surveillance may act as an early alert system for possible outbreaks, as well as provide information about specific areas of the county, or age groups that are being particularly affected.

  • School Absenteeism - An increase in the number of schools reporting high levels of absenteeism, especially during the winter months, generally correlates with the level of influenza in the community.

  • Respiratory Disease Outbreaks at Long Term Care Facilities - Long Term Care Facilities (LTCFs), such as nursing homes and assisted living facilities, are required to report outbreaks of respiratory disease to Public Health.

  • Deaths Due to Pneumonia and Influenza (P&I Deaths) - The proportion of deaths where pneumonia or influenza was listed as an underlying or contributing cause of death (P&I deaths) are reported weekly to Public Health by the King County Vital Statistics office. The proportion of deaths due to P&I each week are compared to the proportion that would be expected in the absence of an influenza epidemic. Seattle also participates in CDC's 122 cities mortality reporting system.

  • Individual Case Reports - Individual deaths due to influenza are reported to Public Health. These cases provide information on risk factors for severe influenza infection, as well as possible vaccine or treatment failures.