2015-2016 Influenza Season Week 40 ending October 10, 2015

All data are preliminary and may change as more reports are received.

Background:

The Centers for Disease Control and Prevention’s (CDC) Influenza Division collects, compiles, and analyzes information on influenza activity year-round in the United States and produces FluView, a weekly influenza surveillance report, and FluView Interactive. The U.S. influenza surveillance system provides information in five categories collected from nine data sources. This is the first report of the 2015-2016 influenza season, which began on October 4, 2015.

The five categories and nine data components of CDC influenza surveillance are:

• Viral Surveillance:U.S. World Health Organization (WHO) collaborating laboratories, the National Respiratory and Enteric Virus Surveillance System (NREVSS), and human infection with novel influenza A virus case reporting;
• Mortality:National Center for Health Statistics (NCHS) Mortality Surveillance System, 122 Cities Mortality Reporting System and influenza-associated pediatric deaths;
• Hospitalizations:Influenza Hospitalization Network (FluSurv-NET) including the Emerging Infections Program (EIP) and three additional states;
• Outpatient Illness Surveillance:U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet);
• Geographic Spread of Influenza:State and territorial epidemiologists’ reports.

An overview of the CDC influenza surveillance system, including methodology and detailed descriptions of each data component, is available at: http://www.cdc.gov/flu/weekly/overview.htm.

Synopsis:

During week 40 (October 4-10, 2015), influenza activity was low in the United States.

• Viral Surveillance: The most frequently identified influenza virus type reported by public health laboratories in week 40 was influenza A viruses, with influenza A (H3) viruses predominating. The percentage of respiratory specimens testing positive for influenza in clinical laboratories is low.
• Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was below their system-specific epidemic threshold in both the NCHS Mortality Surveillance System and the 122 Cities Mortality Reporting System.
• Influenza-associated Pediatric Deaths: No influenza-associated pediatric deaths were reported.
• Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 1.2%, which is below the national baseline of 2.1%. All 10 regions reported ILI below region-specific baseline levels. Georgia experienced low ILI activity; Puerto Rico, New York City and 47 states experienced minimal ILI activity; and the District of Columbia and two states had insufficient data.
• Geographic Spread of Influenza: The geographic spread of influenza in Guam was reported as widespread; one state reported regional activity; one state reported local activity; Puerto Rico and 27 states reported sporadic activity; the U.S. Virgin Islands and 21 states reported no influenza activity; and the District of Columbia did not report.

National and Regional Summary of Select Surveillance Components

*HHS regions (Region 1 CT, ME, MA, NH, RI, VT; Region 2: NJ, NY, Puerto Rico, US Virgin Islands; Region 3: DE, DC, MD, PA, VA, WV; Region 4: AL, FL, GA, KY, MS, NC, SC, TN; Region 5: IL, IN, MI, MN, OH, WI; Region 6: AR, LA, NM, OK, TX; Region 7: IA, KS, MO, NE; Region 8: CO, MT, ND, SD, UT, WY; Region 9: AZ, CA, Guam, HI, NV; and Region 10: AK, ID, OR, WA).
† Elevated means the % of visits for ILI is at or above the national or region-specific baseline
§ Includes all 50 states, the District of Columbia, Guam, Puerto Rico, and U.S. Virgin Islands
‡ National data are for current week; regional data are for the most recent three weeks

U.S. Virologic Surveillance:

WHO and NREVSS collaborating laboratories, which include both public health and clinical laboratories located in all 50 states, Puerto Rico, and the District of Columbia, report to CDC the total number of respiratory specimens tested for influenza and the number positive for influenza virus type. In addition, public health laboratories also report the influenza A subtype (H1 or H3) and influenza B lineage information of the viruses they test and the age or age group of the persons from whom the specimens were collected.

Additional data are available at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html..

The results of tests performed by clinical laboratories during the current week are summarized below.

	Week 40
No. of specimens tested	8,703
No. of positive specimens (%)	104 (1.2%)
Positive specimens by type
Influenza A	76 (73.1%)
Influenza B	28 (26.9%)

View National and Regional Level Graphs and Data | View Chart Data | View Full Screen | View PowerPoint Presentation

The results of tests performed by public health laboratories, as well as the age group distribution of influenza positive tests, during the current week are summarized below.

	Week 40
No. of specimens tested	436
No. of positive specimens	28
Positive specimens by type/subtype
Influenza A	26 (92.9%)
A(H1N1)pmd09	2 (7.7%)
H3	24 (92.3%)
Subtyping not performed	0 (0%)
Influenza B	2 (7.1%)
Yamagata lineage	0 (0%)
Victoria lineage	0 (0%)
Lineage not performed	2 (100%)

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Influenza Virus Characterization:

CDC characterizes influenza viruses through one or more tests including genome sequencing, hemagglutination inhibition (HI) and/or neutralization assays. This data is used to compare how similar currently circulating influenza viruses are to the reference viruses used for developing influenza vaccines, and to monitor for changes in circulating influenza viruses. Historically HI data has been used most commonly to assess the similarity between reference viruses and circulating viruses as a proxy for vaccine effectiveness. Beginning in the 2014–2015 season and to date, however, a portion of influenza A (H3N2) viruses do not yield sufficient hemagglutination titers for antigenic characterization by HI. For many of these viruses, CDC performs genetic characterization to determine the genetic group identity of circulating viruses. In this way, antigenic properties of these viruses can be inferred from viruses within the same genetic group that have been characterized antigenically.

No characterization data is currently available for specimens collected after October 1, 2015.

During May 24-Septemer 30, 2015, CDC has characterized 219 influenza viruses [8 A (H1N1)pdm09, 149 A (H3N2), and 62 influenza B viruses] collected by U.S. laboratories.

Influenza A Virus [157]

• A (H1N1)pdm09 [8]: All 8 influenza A (H1N1)pdm09 viruses were antigenically characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2015-2016 Northern Hemisphere.

• A (H3N2) [149]: All 149 H3N2 viruses were genetically sequenced and all viruses belonged to genetic groups for which a majority of viruses antigenically characterized were similar to A/Switzerland/9715293/2013, the influenza A (H3N2) component of the 2015-2016 Northern Hemisphere vaccine
• A subset of 58 H3N2 viruses also were antigenically characterized; all 58 (100%) H3N2 viruses were A/Switzerland/9715293/2013-like by HI testing or neutralization testing.

Influenza B Virus [62]: Thirty-eight (61%) of the influenza B viruses characterized belonged to B/Yamagata/16/88 lineage and the remaining 24 (39%) influenza B viruses characterized belonged to B/Victoria/02/87 lineage.

Yamagata Lineage [38]: All 38 B/Yamagata-lineage viruses were antigenically characterized as B/Phuket/3073/2013-like, which is included as an influenza B component of the 2015-2016 Northern Hemisphere trivalent and quadrivalent influenza vaccines.

Victoria Lineage [24]: All 24 B/Victoria-lineage viruses were antigenically characterized as B/Brisbane/60/2008-like, the virus that is included as an influenza B component of the 2015-2016 Northern Hemisphere quadrivalent influenza vaccine.

Antiviral Resistance:

No antiviral resistance data is available for specimens collected after October 1, 2015. During May 24-Septemer 30, 2015, 159 specimens (8 influenza A (H1N1)pdm09, 91 influenza A (H3N2), and 60 influenza B viruses) collected in the United States were tested for susceptibility to the neuraminidase inhibitors (oseltamivir, zanamivir, and peramivir). None of the tested viruses were found to be resistant to either oseltamivir, zanamivir, or peramivir.

The majority of recently circulating influenza viruses are susceptible to the neuraminidase inhibitor antiviral medications, oseltamivir, zanamivir, and peramivir; however, rare sporadic instances of oseltamivir-resistant and peramivir-resistant influenza A (H1N1)pdm09 and oseltamivir-resistant influenza A (H3N2) viruses have been detected worldwide. Antiviral treatment is recommended as early as possible for patients with confirmed or suspected influenza who have severe, complicated, or progressive illness; who require hospitalization; or who are at high risk for serious influenza-related complications. Additional information on recommendations for treatment and chemoprophylaxis of influenza virus infection with antiviral agents is available at http://www.cdc.gov/flu/antivirals/index.htm.

Pneumonia and Influenza (P&I) Mortality Surveillance:

Rapid tracking of pneumonia and influenza-associated deaths is done through two systems, the National Center for Health Statistics (NCHS) Mortality Surveillance System and the 122 Cities Mortality Reporting System. NCHS mortality surveillance data are presented by the week the death occurred and P&I percentages are released two weeks after the week of death to allow for collection of enough data to produce a stable P&I percentage. Users of the data should not expect the two systems to produce the same percentages, and the percent P&I deaths from each system should be compared to the corresponding system-specific baselines and thresholds.

NCHS Mortality Surveillance Data:

Based on NCHS mortality surveillance data available on October 15, 2015, 5.5% of the deaths occurring during the week ending September 26, 2015 (week 38) were due to P&I. This percentage is below the epidemic threshold of 6.2% for week 38.

Region and state-specific data are available at http://www.cdc.gov/flu/weekly/nchs.htm.

View Regional and State Level Data | View Chart Data | View Full Screen | View PowerPoint Presentation

122 Cities Mortality Reporting System:

During week 40, 5.3% of all deaths reported through the 122 Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 5.9% for week 40.

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Influenza-Associated Pediatric Mortality:

No influenza-associated pediatric deaths were reported to CDC during week 40.

Additional data can be found at: http://gis.cdc.gov/GRASP/Fluview/PedFluDeath.html.

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Influenza-Associated Hospitalizations:

The Influenza Hospitalization Surveillance Network (FluSurv-NET) conducts all age population-based surveillance for laboratory-confirmed influenza-related hospitalizations in select counties in the Emerging Infections Program (EIP) states and Influenza Hospitalization Surveillance Project (IHSP) states. FluSurv-NET estimated hospitalization rates will be updated weekly starting later this season. Additional FluSurv-NET data can be found at: http://gis.cdc.gov/GRASP/Fluview/FluHospRates.html and http://gis.cdc.gov/grasp/fluview/FluHospChars.html.

Outpatient Illness Surveillance:

Nationwide during week 40, 1.2% of patient visits reported through the U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet) were due to influenza-like illness (ILI). This percentage is below the national baseline of 2.1%.

(ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and cough and/or sore throat.)

Additional data are available at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.

View National and Regional Level Graphs and Data | View Chart Data | View Full Screen | View PowerPoint Presentation

On a regional level, the percentage of outpatient visits for ILI ranged from 0.4% to 3.2% during week 40. All 10 regions reported a proportion of outpatient visits for ILI below their region-specific baseline levels.

ILINet State Activity Indicator Map:

Data collected in ILINet are used to produce a measure of ILI activity* by state. Activity levels are based on the percent of outpatient visits in a state due to ILI and are compared to the average percent of ILI visits that occur during weeks with little or no influenza virus circulation. Activity levels range from minimal, which would correspond to ILI activity from outpatient clinics being below, or only slightly above, the average, to high, which would correspond to ILI activity from outpatient clinics being much higher than average.

During week 40, the following ILI activity levels were calculated:

• One state experienced low ILI activity (Georgia).
• New York City, Puerto Rico and 47 states (Alabama, Alaska, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Hawaii, Idaho, Illinois, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, Nebraska, Nevada, New Hampshire, New Mexico, New York, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, South Dakota, Tennessee, Texas, Utah, Virginia, Vermont, Washington, West Virginia, Wisconsin, and Wyoming) experienced minimal ILI activity.
• Data were insufficient to calculate an ILI activity level from the District of Columbia and two states (Arizona and New Jersey).

 

*This map uses the proportion of outpatient visits to health care providers for ILI to measure the ILI activity level within a state. It does not, however, measure the extent of geographic spread of flu within a state. Therefore, outbreaks occurring in a single city could cause the state to display high activity levels.
Data collected in ILINet may disproportionally represent certain populations within a state, and therefore, may not accurately depict the full picture of influenza activity for the whole state.
Data displayed in this map are based on data collected in ILINet, whereas the State and Territorial flu activity map is based on reports from state and territorial epidemiologists. The data presented in this map is preliminary and may change as more data are received.
Differences in the data presented here by CDC and independently by some state health departments likely represent differing levels of data completeness with data presented by the state likely being the more complete.

Geographic Spread of Influenza as Assessed by State and Territorial Epidemiologists

The influenza activity reported by state and territorial epidemiologists indicates geographic spread of influenza viruses, but does not measure the severity of influenza activity.

During week 40, the following influenza activity was reported:

• Widespread influenza activity was reported by Guam.
• Regional influenza activity was reported by one state (Hawaii).
• Local influenza activity was reported by one state (Oklahoma).
• Sporadic influenza activity was reported by the Puerto Rico and 27 states (Alaska, Arizona, California, Connecticut, Florida, Georgia, Kentucky, Louisiana, Maine, Massachusetts, Michigan, Minnesota, Missouri, Montana, New York, North Carolina, North Dakota, Ohio, Oregon, South Carolina, Texas, Utah, Vermont, Washington, West Virginia, Wisconsin, and Wyoming).
• No influenza activity was reported by the U.S. Virgin Islands and 21 states (Alabama, Arkansas, Colorado, Delaware, Idaho, Illinois, Indiana, Iowa, Kansas, Maryland, Mississippi, Nebraska, Nevada, New Hampshire, New Jersey, New Mexico, Pennsylvania, Rhode Island, South Dakota, Tennessee, and Virginia).
• The District of Columbia did not report.