County of Marin Health and Human Services

Influenza Surveillance Update, Two weeks ending March 21

Marin

During week 10 (March 8-14) and week 11 (March 15-21), influenza-like illness activity in Marin County was below peak levels, but remained elevated compared to baseline levels (Figure 1). The number and percent of positive influenza tests continued decreasing, with only two positive tests reported in week 11 (Figure 2). Due to the substantial reduction in positive influenza tests and anticipated decline in influenza-like illness, this report is the last of the 2014-2015 influenza season.

Influenza-like illness activity

Influenza-like illness (ILI) in Marin County was moderate during week 10 (March 8-14) and week 11 (March 15-21). Discharge data from Kaiser San Rafael, Marin General Hospital, and Novato Community Hospital emergency departments and visits to Sutter Terra Linda Urgent Care showed the percentage of visits associated with ILI was 12.8% during week 10 and 13.1% during week 11. See Figure 1 below.

Figure 1. Proportion of Visits due to Influenza-Like Illness (ILI): Kaiser San Rafael, MGH, and NCH Emergency Departments and Sutter Terra Linda Urgent Care, 2012-13, 2013-14, and 2014-15 Influenza SeasonsInfluenza-like illness, Marin County, 2012-2015

PROVISIONAL DATA, SUBJECT TO CHANGE. The case definition for ILI is any chief complaint field that includes the following keywords or variants of the keywords: influenza, flu, influenza-like illness, pneumonia, fever, cough, and/or sore throat.

We updated the influenza-like illness (ILI) syndrome definition by broadening the list of chief complaints. As a result, the weekly percentages of visits for ILI are higher throughout the year than was seen using the prior definition, but the same general patterns from previous seasons remain. We applied the improved definition to all seasons in the graph above.

Influenza testing

During week 10, 6 (3.6%) of the 166 specimens tested for influenza were positive. During week 11, 2 (1.3%) of the 152 specimens tested for influenza were positive. Half of the positive specimens during week 10 were influenza A and the remaining specimens were influenza B. All positive specimens during week 11 were influenza A. As of March 21, 248 positive tests have been reported for the 2014-2015 influenza season. See Figure 2 below.

Figure 2. Influenza Tests in Marin County Reported by: Kaiser Novato, Kaiser San Rafael, Marin General Hospital, Novato Community Hospital, and Sutter Terra Linda Urgent Care, 2014-15 Influenza Season
Influenza Tests in Marin County, 2014-2015

Visit MarinFlu.org for influenza vaccination, testing, and treatment recommendations.

California

  • Overall influenza activity in California was regional* in week 10 and local^ in week 11.
  • Outpatient ILI activity as a percentage of total visits to sentinel providers was 3.1% in week 10 and 2.2% in week 11.
  • Of the 2,785 specimens tested by the Respiratory Laboratory Network and Sentinel Laboratories during week 10, 264 (9.5%) were positive for influenza; of these, 93 (35.2%) were positive for influenza B and 171 (64.8%) were positive for influenza A. Of the influenza A specimens, 46 (26.9%) were subtyped as seasonal A (H3), none were subtyped as 2009 A (H1), and 125 (73.1%) were not subtyped. Of the 2,324 specimens tested during week 11, 160 (6.9%) were positive for influenza; of these, 67 (41.9%) were positive for influenza B and 93 (58.1%) were positive for influenza A. Of the influenza A specimens, 19 (20.4%) were subtyped as seasonal A (H3), none were subtyped as 2009 A (H1), and 74 (79.6%) were not subtyped.
  • Kaiser Permanente hospitalization data for northern and southern California showed that the percentage of hospitalizations for flu admissions, defined as a diagnosis of "flu," "influenza," or "pneumonia" recorded in text fields at time of admission, was 6.6% during week 10 and 5.9% during week 11.
  • California Department of Public Health has received 50 reports of influenza-associated deaths in persons under age 65 during the 2014-2015 influenza season to date.
  • The California Department of Public Health's Influenza website contains guidance and additional data.

United States

  • Influenza activity in the United States continued to decrease, but remained elevated in weeks 10 and 11.
  • Some circulating H3N2 strains differ from the H3N2 component of this season's influenza vaccine. The vaccine is still recommended. CDC released additional guidance on antiviral treatment.
  • Of the 15,033 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week 10, 1,685 (11.2%) were positive for influenza. During week 11, 1,358 (10.6%) tests were positive for influenza of 12,824 tests conducted.
  • The proportion of deaths due to pneumonia and influenza (P&I) was above the epidemic threshold in weeks 10 and 11.
  • During week 10, three influenza-associated pediatric deaths were reported. During week 11, nine influenza-associated pediatric deaths were reported.
  • The proportion of outpatient visits for influenza-like illness (ILI) was 2.3% during week 10 and 2.2% during week 11.
  • During week 10, eight regions reported a proportion of outpatient visits for ILI at or above the region-specific baseline level. ILI activity was high in four states, moderate in six states, low in six states, and minimal in New York City and 34 states. During week 11, six regions reported a proportion of outpatient visits for ILI at or above the region-specific baseline level. ILI activity was high in one state, moderate in four states, low in 12 states, and minimal in New York City and 33 states.
  • During week 10, the geographic spread of influenza activity was reported as widespread+ in seven states, regional* in 29 states, local^ in 13 states, and sporadic~ in one state. During week 11, the geographic spread of influenza activity was reported as widespread+ in seven states, regional* in 27 states, local^ in 15 states, and sporadic~ in one state. See map below for week 11.
  • For additional national influenza data, visit the Center for Disease Control and Prevention's influenza surveillance website.

Geographic Distribution of Influenza in the US

+Widespread: Outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of the state with recent laboratory evidence of influenza in the state.
*Regional: Outbreaks of influenza or increases in ILI and recent laboratory confirmed influenza in at least two but less than half the regions of the state with recent laboratory evidence of influenza in those regions
^Local: Outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of the state.
~Sporadic: Small numbers of laboratory-confirmed influenza cases or a single laboratory-confirmed influenza outbreak has been reported, but there is no increase in cases of ILI.

Additional flu trends can be seen in Google Flu Trends which uses aggregated Google search data in a model created in collaboration with CDC to estimate influenza activity in the United States. For more information and activity estimates from California, the U.S. and worldwide, see http://www.google.org/flutrends/

If you have any questions or comments please contact:
Jessica Cunningham-Krahl, MPH
Epidemiologist
(415) 473-7346
JCunningham@marincounty.org

This publication is provided to you by the Office of the Public Health Officer, County of Marin Department of Health and Human Services.

Sources

Centers for Disease Control and Prevention. https://www.cdc.gov/flu/weekly/ and https://www.cdc.gov/flu/weekly/weeklyarchives2014-2015/week10.htm. Accessed 3/31/2015.

California Department of Public Health. https://www.cdph.ca.gov/data/statistics/Pages/CISPDataArchive.aspx. Accessed 3/31/2015.

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