2013-2014 Influenza Season Week 52 ending December 28, 2013

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

Synopsis:

During week 52 (December 22-28, 2013), influenza activity continued to increase in the United States.

• Viral Surveillance: Of 6,419 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories during week 52, 1,711 (26.7%) were positive for influenza.
• Pneumonia and Influenza Mortality: The proportion of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold.
• Influenza-Associated Pediatric Deaths: Two influenza-associated pediatric deaths were reported.
• Influenza-associated Hospitalizations: A cumulative rate for the season of 5.8 laboratory-confirmed influenza-associated hospitalizations per 100,000 population was reported.
• Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 4.3%, above the national baseline of 2.0%. All 10 regions reported ILI at or above region-specific baseline levels. Twenty states experienced high ILI activity; eight states and New York City experienced moderate ILI activity; six states experienced low ILI activity; 16 states experienced minimal ILI activity, and the District of Columbia had insufficient data.
• Geographic Spread of Influenza: The geographic spread of influenza in 25 states was reported as widespread; 19 states reported regional influenza activity; Puerto Rico and three states reported local influenza activity; the District of Columbia and two states reported sporadic influenza activity, and the U.S. Virgin Islands and Guam did not report.

A description of surveillance methods is available at: http://www.cy118119.com/flu/weekly/overview.htm

*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
‡ National data are for current week; regional data are for the most recent three weeks
§ Includes all 50 states, the District of Columbia, Guam, Puerto Rico, and U.S. Virgin Islands

U.S. Virologic Surveillance

WHO and NREVSS collaborating laboratories located in all 50 states, Puerto Rico, and Washington D.C. report to CDC the number of respiratory specimens tested for influenza and the number positive by influenza virus type and influenza A virus subtype. The results of tests performed during the current week are summarized in the table below.

Region specific data can be found at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.

	Week 52
No. of specimens tested	6,419
No. of positive specimens (%)	1,711 (26.7%)
Positive specimens by type/subtype
Influenza A	1,667 (97.4%)
2009 H1N1	915 (54.9%)
H3	7 (0.4%)
Subtyping not performed	745 (44.7%)
Influenza B	44 (2.6%)

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Antigenic Characterization*

CDC has antigenically characterized 452 influenza viruses [398 2009 H1N1 viruses, 46 influenza A (H3N2) viruses, and 8 influenza B viruses] collected by U.S. laboratories since October 1, 2013 by hemagglutination inhibition (HI).

2009 H1N1 [398]:

• All 398 2009 H1N1 viruses tested were characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2013-2014 Northern Hemisphere influenza vaccine.

Influenza A (H3N2) [46]:

• All 46 influenza A (H3N2) viruses tested have been characterized as A/Texas/50/2012-like, the influenza A (H3N2) component of the 2013-2014 Northern Hemisphere influenza vaccine.

Influenza B [8]: Three (38%) of the eight influenza B viruses tested belong to B/Yamagata/16/88-lineage and the remaining five (62%) influenza B viruses tested belong to B/Victoria/02/87 lineage.

• Yamagata Lineage [3]: Three influenza B/Yamagata-lineage viruses were characterized as B/ Massachusetts/2/1012-like, which is included as an influenza B component of the 2013-2014 Northern Hemisphere trivalent and quadrivalent influenza vaccines.
• Victoria Lineage [5]: Five influenza B/Victoria-lineage viruses were characterized as B/Brisbane/60/2008-like, which is included as an influenza B component of the 2013-2014 Northern Hemisphere quadrivalent influenza vaccine.

*For more information see the section on antigenic characterization in the MMWR “Update: Influenza Activity — United States and Worldwide, May 19–September 28, 2013”.

Antiviral Resistance

Testing of 2009 H1N1, influenza A (H3N2), and influenza B virus isolates for resistance to neuraminidase inhibitors (oseltamivir and zanamivir) is performed at CDC using a functional assay. Additional 2009 H1N1 and influenza A (H3N2) clinical samples are tested for mutations of the virus known to confer oseltamivir resistance. The data summarized below combine the results of both testing methods. These samples are routinely obtained for surveillance purposes rather than for diagnostic testing of patients suspected to be infected with antiviral-resistant virus.

High levels of resistance to the adamantanes (amantadine and rimantadine) persist among 2009 influenza A (H1N1) and A (H3N2) viruses (the adamantanes are not effective against influenza B viruses). As a result, data from adamantane resistance testing are not presented below.

Neuraminidase Inhibitor Resistance Testing Results on Samples Collected Since October 1, 2013

	Oseltamivir		Zanamivir
	Virus Samples tested (n)	Resistant Viruses, Number (%)	Virus Samples tested (n)	Resistant Viruses, Number (%)
Influenza A (H3N2)	66	0 (0.0)	66	0 (0.0)
Influenza B	15	0 (0.0)	15	0 (0.0)
2009 H1N1	910*	10 (1.1)	482	0 (0.0)

*Includes specimens tested in national surveillance and additional specimens tested at public health laboratories in 11 states (AZ, CO, FL, GA, HI, MD, MI, NY, TX, WA, and WI) who share testing results with CDC.

The majority of currently circulating influenza viruses are susceptible to the neuraminidase inhibitor antiviral medications, oseltamivir and zanamivir; however, rare sporadic cases of oseltamivir-resistant 2009 H1N1 and A (H3N2) viruses have been detected worldwide. Antiviral treatment with oseltamivir or zanamivir 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 greater 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.cy118119.com/flu/antivirals/index.htm.

Pneumonia and Influenza (P&I) Mortality Surveillance

During week 52, 6.5% of all deaths reported through the 122 Cities Mortality Reporting System were due to P&I. This percentage was below the epidemic threshold of 7.0% for week 52.

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Influenza-Associated Pediatric Mortality

Two influenza-associated pediatric deaths were reported to CDC during week 52. One death was associated with a 2009 H1N1 virus and occurred during week 51 (week ending December 21, 2013) and one was associated with an influenza A virus for which no subtyping was performed and occurred during week 50 (week ending December 14, 2013). A total of six influenza-associated pediatric deaths for the 2013-2014 season have been reported.

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 population-based surveillance for laboratory-confirmed influenza-related hospitalizations in children younger than 18 years of age (since the 2003-2004 influenza season) and adults (since the 2005-2006 influenza season).

The FluSurv-NET covers more than 70 counties in the 10 Emerging Infections Program (EIP) states (CA, CO, CT, GA, MD, MN, NM, NY, OR, TN) and additional Influenza Hospitalization Surveillance Project (IHSP) states. The IHSP began during the 2009-2010 season to enhance surveillance during the 2009 H1N1 pandemic. IHSP sites included IA, ID, MI, OK and SD during the 2009-2010 season; ID, MI, OH, OK, RI, and UT during the 2010-2011 season; MI, OH, RI, and UT during the 2011-2012 season; IA, MI, OH, RI, and UT during the 2012-2013 season; and MI, OH, and UT during the 2013-2014 season.

Data gathered are used to estimate age-specific hospitalization rates on a weekly basis, and describe characteristics of persons hospitalized with severe influenza illness. The rates provided are likely to be an underestimate as influenza-related hospitalizations can be missed, either because testing is not performed, or because cases may be attributed to other causes of pneumonia or other common influenza-related complications.

Between October 1, 2013 and December 28, 2013, 1,583 laboratory-confirmed influenza-associated hospitalizations were reported. This is a rate of 5.8 per 100,000 population. Among cases, 1,492 (94.3%) were influenza A, 76 (4.8%) were influenza B, 8 (0.5%) were influenza A and B co-infection, and 7 (0.4%) had no virus type information. Among those with influenza A subtype information, 8 (1.2%) were H3 and 660 (98.2%) were 2009 H1N1. The most commonly reported underlying medical conditions among adults were obesity, metabolic disorders, cardiovascular disease, chronic lung disease (excluding asthma), and asthma. The most commonly reported underlying medical conditions in children were asthma, obesity, neurologic disorders, and chronic lung disease. Approximately 43.5% of hospitalized children had no identified underlying medical conditions. Among 60 hospitalized women of childbearing age (15-44 years), twelve were pregnant.

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.

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Outpatient Illness Surveillance

Nationwide during week 52, 4.3% 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 above the national baseline of 2.0%. (ILI is defined as fever (temperature of 100°F [37.8°C] or greater) and cough and/or sore throat.)

The increase in the percentage of patient visits for ILI in previous weeks may be influenced in part by a reduction in routine healthcare visits during the holidays, as has occurred in previous seasons.

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On a regional level, the percentage of outpatient visits for ILI ranged from 1.2% to 9.2% during week 52. All 10 regions reported a proportion of outpatient visits for ILI at or above their region-specific baseline level.

Region specific data is available at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.

ILINet 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 52, the following ILI activity levels were experienced:

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

 

*This map uses the proportion of outpatient visits to health care providers for influenza-like illness 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 disproportionately 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 is 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 52, the following influenza activity was reported:

• Widespread influenza activity was reported by 25 states (Alabama, Alaska, Arkansas, Colorado, Connecticut, Georgia, Idaho, Illinois, Indiana, Kansas, Louisiana, Massachusetts, Minnesota, Nebraska, Nevada, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Texas, Utah, Virginia, Washington, and Wyoming).
• Regional influenza activity was reported by 20 states (California, Florida, Iowa, Kentucky, Maine, Maryland, Michigan, Mississippi, Missouri, Montana, New Hampshire, New Jersey, New Mexico, North Dakota, Ohio, South Carolina, South Dakota, Tennessee, West Virginia, and Wisconsin).
• Local influenza activity was reported by Puerto Rico and three states (Arizona, Delaware, and Vermont).
• Sporadic influenza activity was reported by the District of Columbia and two states (Hawaii and Rhode Island).
• The U.S. Virgin Islands and Guam did not report.