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Public Health - Seattle & King County

Influenza or the "flu"

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, neurological dysfunction, and obesity. Annual influenza vaccination reduces the risk for illness and is recommended for all persons six months of age and older. Influenza is spread primarily by respiratory droplets (droplet spread) produced when an infected person coughs or sneezes.

Resources for the general public and health care providers
Influenza in King County

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 from laboratory-confirmed influenza

Local epidemiology

See current 2015-16 flu season weekly surveillance reports

Influenza Season Summary, 2014-2015:

As of 3/30/2015, influenza activity had returned to baseline levels for two consecutive weeks in King County. The 2014-15 influenza season was more severe than the past four flu seasons, with a record number of reported laboratory-confirmed influenza deaths and influenza outbreaks in long-term care facilities (LTCFs). As defined by rises from baseline across multiple influenza activity indicators, the onset of the 2014-15 season occurred in the last week of November 2014. Peak influenza activity based on emergency department (ED) influenza-like illness (ILI) and laboratory reporting occurred from late December through January, which was similar to the past two influenza seasons. The predominant strain was influenza A (H3N2) compared with A (H1N1) in 2013-14.

Because most persons with severe influenza-related illness are not tested for influenza, routine surveillance data is most useful for tracking trends and unusual disease patterns and not as an indicator of the total number of influenza-related deaths or influenza infections. Special studies are done in representative communities nationally to determine hospitalization and death rates from influenza.

  • Influenza deaths: A total of 34 laboratory-confirmed influenza-related deaths have been reported in King County; this is higher than in any of the past five seasons (including the 2009-10 pandemic season), where number of deaths reported has ranged from seven to 24. Forty-one percent of cases were male, and 82% were aged 65 or older (range: 30 – 105 years, median: 82 years). There were no reported pediatric deaths. With the exception of one influenza B case, all cases were attributable to influenza A [6 A(H3), 27 A (not typed)]. Nearly all had severe underlying conditions, including chronic cardiac, kidney and respiratory disorders, and diabetes, and 59% had no evidence of influenza vaccine for this season. Estimates have indicated that between 65-256 influenza deaths are likely to occur in King County each flu season, but many go undiagnosed or unreported; during the 2014-15 flu season, it is likely that total influenza-attributable deaths were at the upper end of this range.

  • Outbreaks in long-term care facilities (LTCF): Sixty-three outbreaks were reported from 59 LTCF, all of which identified at least one laboratory-confirmed case of influenza. This was higher than in any of the past five flu seasons, where number of LTCF outbreaks reported has ranged from seven to 55. One third of this season's LTCF outbreaks were reported during the first week of January, during peak activity. Seventeen LTCF outbreaks resulted in one or more deaths.

  • Laboratory: King County Public Health Laboratory tested a total of 223 specimens contributed by sentinel influenza providers, 31% of which were positive for influenza. Of the 70 positive specimens, 62 (89%) were typed as A (H3), four (6%) were A (not typed), and four (6%) were influenza B.

  • Syndromic surveillance: The peak volume of ED visits for ILI was approximately 5.4% - higher than that observed in the past five years, excluding the pandemic H1N1 period. Peak activity occurred during the last few weeks of December 2014. ED volume was highest among pediatric age groups with peak visit levels at approximately 20% among children aged two to four years old and 15% among infants under two years old.

  • Influenza vaccine effectiveness: The National Vaccine Effectiveness (VE) Network estimate (data through January 30, 2015) for influenza A H3N2 viruses was 18% (95% CI: 6%-29%), and for influenza B was 45% (95% CI: 14%-65%). Across all strains, the combined vaccine effectiveness is estimated at 19% (95% CI: 7%-29%). Reduced protection against H3N2 viruses this season has been attributed to the fact that more than two-thirds of circulating H3N2 viruses are drifted from the H3N2 vaccine virus recommended for vaccine production. The proportion of drifted viruses at the U.S. VE study sites was even higher (>80%).

More information

For King County health care providers: