County of Marin Health and Human Services

Influenza Surveillance Update, Two weeks ending March 7

Marin

During week 8 (February 22-28) and week 9 (March 1-7), influenza-like illness activity in Marin County decreased but remained elevated compared to baseline levels (Figure 1). A decline in the number and percent of positive influenza tests began in early February (Figure 2).

Influenza-like illness activity

Influenza-like illness (ILI) in Marin County was moderate during week 8 (February 22-28) and week 9 (March 1-7). 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 11.9% during week 8 and 12.5% during week 9. 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 8, 14 (7.9%) of the 177 specimens tested for influenza were positive. During week 9, 10 (5.8%) of the 173 specimens tested for influenza were positive. The majority of specimens during weeks 8 and 9 were influenza A (92.9% and 80.0%, respectively) and the remaining specimens were influenza B. As of March 7, 240 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 widespread+ in week 8 and regional* in week 9.
  • Outpatient ILI activity as a percentage of total visits to sentinel providers was 3.1% in week 8 and 2.4% in week 9.
  • Of the 3,493 specimens tested by the Respiratory Laboratory Network and Sentinel Laboratories during week eight, 396 (11.3%) were positive for influenza; of these, 65 (16.4%) were positive for influenza B and 331 (83.6%) were positive for influenza A. Of the influenza A specimens, 113 (34.1%) were subtyped as seasonal A (H3), 2 (0.6%) were subtyped as 2009 A (H1), and 216 (65.3%) were not subtyped. Of the 2,761 specimens tested during week nine, 307 (11.1%) were positive for influenza; of these, 65 (21.2%) were positive for influenza B and 242 (78.8%) were positive for influenza A. Of the influenza A specimens, 105 (43.4%) were subtyped as seasonal A (H3), none were subtyped as 2009 A (H1), and 137 (56.6%) 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 7.7% during week 8 and 7.2% during week 9.
  • California Department of Public Health has received 45 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 decreased, but remained elevated in week 8 and week 9.
  • 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 16,821 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week eight, 1,834 (10.9%) were positive for influenza. During week nine, 1,670 (11.4%) tests were positive for influenza of 14,634 tests conducted.
  • The proportion of deaths due to pneumonia and influenza (P&I) was at or above the epidemic threshold in weeks 8 and 9.
  • During week 8, six influenza-associated pediatric deaths were reported. During week 9, seven influenza-associated pediatric deaths were reported.
  • The proportion of outpatient visits for influenza-like illness (ILI) was 2.5% during week 8 and 2.4% during week 9.
  • During week 8, seven regions reported a proportion of outpatient visits for ILI at or above the region-specific baseline level. ILI activity was high in six states, moderate in four states, low in ten states, and minimal in New York City and 30 states. During week 9, eight regions reported a proportion of outpatient visits for ILI at or above the region-specific baseline level. ILI activity was high in six states, moderate in one state, low in 13 states, and minimal in New York City and 30 states.
  • During week 8, the geographic spread of influenza activity was reported as widespread+ in 12 states, regional* in 30 states, local^ in six states, and sporadic~ in two states. During week 9, the geographic spread of influenza activity was reported as widespread+ in nine states, regional* in 29 states, local^ in 11 states, and sporadic~ in one state. See map below for week 9.
  • 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/week8.htm. Accessed 3/18/2015.

California Department of Public Health. https://www.cdph.ca.gov/data/statistics/Pages/CISPDataArchive.aspx. Accessed 3/18/2015.

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