2012-2013 Influenza Season Week 52 ending December 29, 2012

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

Synopsis:

During week 52 (December 23-29), influenza activity increased in the U.S.

• Viral Surveillance: Of 9,363 specimens tested and reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories, 2,961 (31.6%) 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 and were associated with influenza B viruses.
  
• Outpatient Illness Surveillance: The proportion of outpatient visits for influenza-like illness (ILI) was 5.6%; above the national baseline of 2.2%. Nine of 10 regions reported ILI above region-specific baseline levels. New York City and 29 states experienced high ILI activity; 9 states experienced moderate ILI activity; 4 states experienced low ILI activity; 6 states experienced minimal ILI activity, and the District of Columbia and 2 states had insufficient data.
  
• Geographic Spread of Influenza: Forty-one states reported widespread geographic influenza activity; 7 states reported regional activity; the District of Columbia reported local activity; 1 state reported sporadic activity; Guam reported no influenza activity, and Puerto Rico, the U.S. Virgin Islands, and 1 state did not report.

A description of surveillance methods is available at: http://www.cdc.gov/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 and Puerto Rico report to CDC the number of respiratory specimens tested for influenza and the number positive by influenza virus type and influenza A virus subtype. Region specific data can be found at http://gis.cdc.gov/grasp/fluview/fluportaldashboard.html.

	Week 52
No. of specimens tested	9,363
No. of positive specimens (%)	2,961 (31.6%)
Positive specimens by type/subtype
Influenza A	2,346 (79.2%)
2009 H1N1	25 (1.1%)
Subtyping not performed	1,112 (47.4%)
H3	1,209 (51.5%)
Influenza B	615 (20.8%)

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

Since the start of the season, influenza A (H3N2) viruses have predominated nationally, followed by influenza B viruses, while 2009 H1N1 viruses have been identified rarely. The predominant circulating virus has varied by state and by region.

Antigenic Characterization:

CDC has antigenically characterized 413 influenza viruses [17 2009 H1N1 viruses, 281 influenza A (H3N2) viruses, and 115 influenza B viruses] collected by U.S. laboratories since October 1, 2012.

2009 H1N1 [17]:

• • All 17 2009 H1N1 viruses tested were characterized as A/California/7/2009-like, the influenza A (H1N1) component of the 2012-2013 influenza vaccine for the Northern Hemisphere.

Influenza A (H3N2) [281]:

• 279 (99.3%) of the 281 H3N2 influenza viruses tested have been characterized as A/Victoria/361/2011-like, the influenza A (H3N2) component of the 2012-2013 Northern Hemisphere influenza vaccine.
  
• 2 (0.7%) of the 281 H3N2 viruses tested showed reduced titers with antiserum produced against A/Victoria/361/2011.

Influenza B (B/Yamagata/16/88 and B/Victoria/02/87 lineages) [115]:

• Yamagata Lineage [79]: 79 (68.7%) of the 115 influenza B viruses tested so far this season have been characterized as B/Wisconsin/1/2010-like, the influenza B component of the 2012-2013 Northern Hemisphere influenza vaccine.
  
  
  
• Victoria Lineage [36]: 36 (31.3%) of 115 influenza B viruses tested have been from the B/Victoria lineage of viruses.
  

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 influenza A (H1N1) clinical samples are tested for a single mutation in the neuraminidase of the virus known to confer oseltamivir resistance (H275Y). 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, 2012

	Oseltamivir		Zanamivir
	Virus Samples tested (n)	Resistant Viruses, Number (%)	Virus Samples tested (n)	Resistant Viruses, Number (%)
Influenza A (H3N2)	526	0 (0.0)	526	0 (0.0)
Influenza B	226	0 (0.0)	226	0 (0.0)
2009 H1N1	39	0 (0.0)	30	0 (0.0)

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