Flu Season Activity in King County

Learn about Public Health – Seattle & King County’s Influenza Surveillance Program.

Weekly snapshot
Influenza is circulating at low levels in the community; all surveillance indicators suggest that flu activity peaked and has been declining for the past several weeks. Several long-term care facilities have reported influenza outbreaks since the beginning of March, with influenza B predominating.

Sentinel Provider Surveillance through the Public Health Laboratory:

During week 15 (ending 4/12/2008), sentinel providers submitted 19 specimens for testing, 2 of which were positive for influenza.

Since September 30, 2007, the PHSKC lab has received 583 specimens for respiratory virus culture, yielding

87 influenza A (H1)
44 influenza A (H3)
37 influenza A (not typed)
64 influenza B/Yamagata lineage
2 influenza B/Victoria lineage
7 influenza B (not typed)
7 adenovirus
1 parainfluenza type 1
5 respiratory syncytial virus (RSV)

Specimens that test positive for influenza are forwarded to the Centers for Disease Control and Prevention (CDC) laboratories for further antigenic characterization. CDC has characterized 9 King County influenza A (H1) isolates from November and December 2007 as A/Solomon Islands/03/2006-like (H1N1) (contained in 2007-08 vaccine), 1 influenza A (H1) isolate from December 2007 as A/Solomon Islands/03/2006-like (H1N1) low (reduced coverage by vaccine), and 2 influenza A (H3) isolates from December 2007 as A/Brisbane/10/2007-like (H3N2). A/Brisbane/10/2007 is a recent antigenic variant which evolved from A/Wisconsin/67/2005-like. A/Brisbane/10/2007-like virus is the recommended influenza A (H3) component for the 2008 Southern Hemisphere vaccine.

To compare this year’s influenza season with previous seasons, click to enlarge the images below.

During week 15, 15% of rapid influenza antigen tests at hospital microbiology labs were positive for influenza (click on table to view data by week). The peak in proportion of tests positive for influenza occurred during week 4 at 30% and has declined since then. Overall, rapid antigen test results this season-to-date are comparable to the mean level for the previous four seasons. For information on respiratory virus detections visit University of Washington’s Clinical Virology Lab.

Syndromic Surveillance:
The proportion of emergency department (ED) visits for febrile illness and for influenza-like illness decreased during week 15. ED visits for febrile illness and ILI peaked during weeks 5 through 9. Compared to the previous 3 seasons, the proportion of ED visits attributed to ILI has increased among children under the age of five, and is comparable to previous years for other age groups.

P & I (Pneumonia and Influenza) Deaths:
King County P&I deaths have exceeded the national epidemic threshold seven times during the 2007-08 season, most recently during week fourteen, and returned to within normal limits the subsequent week. Nationally, P&I deaths have exceeded the national epidemic threshold for thirteen consecutive weeks. No pediatric influenza deaths have been reported in King County during the 2007-08 season.

Long-term Care Facilities:
Thirteen LTCF have reported outbreaks of ILI this season-to-date, eight of which were reported during the month of March. Five facilities reported positive influenza A test results and five reported influenza B. Other facilities had negative influenza testing.

School Absenteeism:
For the 2007-08 school year, King County schools are being asked to report weekly absenteeism online, whether or not absenteeism exceeds 10% in a given week. Absenteeism greater than 10% peaked during weeks 5 and 6 and has since started to decline. Overall, the absenteeism reports to date exceed those observed in previous years, but may in part be due to a change in the reporting mechanism.

During week 14 (March 30 – April 5, 2008), influenza activity continued to decrease in the United States. Three hundred seventy-six (13.2%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza. The proportion of deaths attributed to pneumonia and influenza has been above the epidemic threshold for 13 consecutive weeks. The proportion of outpatient visits for influenza-like illness (ILI) and the proportion of outpatient visits for acute respiratory illness (ARI) were below national baseline levels. ILI decreased in all nine regions compared to week 13, but remained above the region-specific baselines in the East North Central and West North Central regions. The proportion of outpatient visits reported for ARI was below all region and age-specific baselines. Six states reported widespread influenza activity; 11 states reported regional influenza activity; 23 states reported local influenza activity; and 10 states and the District of Columbia reported sporadic influenza activity.

Since September 30, 2007, 18 counties have reported 901 positive influenza isolates. Forty-Four (4.8%) were influenza A (H3N2), 100 (11%) were A, (H1N1), 436 (48.3%) were A, (Not Typed), and 321 (35.9%) were influenza B. A majority of the influenza B isolates looked at further were influenza B, Yamagata (not in this year’s vaccine). Isolation submission peaked the week ending February 2, 2008 with 12% of all isolates received. Submissions received have been falling steadily for the past four weeks with last weeks isolates received accounting for only 3.8% of all isolates received. Eighteen nursing home/adult living facilities have reported influenza outbreaks this season. Ten have been influenza B outbreaks and 8 have been influenza A. Influenza activity in Washington State is being reported as Local to the Centers for Disease Control (CDC).

Influenza activity remained similar to previous weeks and has become predominantly localized throughout BC. The number of isolates and percent positive for influenza remained consistent. The proportion of influenza B reported continued to decrease in week 14, whereas influenza A has remained steady. In week 14 each strain made up approximately 50% of the isolates reported as influenza. Reports of outbreaks remained low. The rate of ILI visits to physicians decreased in week 14 but remained within the expected range for this point in the season.