http://www.cdc.gov/flu/weekly/

Weekly Report: Influenza Summary Update

2007-2008 Influenza Season Week 12, ending March 22, 2008

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

Synopsis

During week 12 (March 16 – 22, 2008), influenza activity continued to decrease in the United States.

• Nine hundred eighty-six (21.3%) specimens tested by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories were positive for influenza.
• The proportion of deaths attributed to pneumonia and influenza was above the epidemic threshold for the eleventh consecutive week.
• The proportion of outpatient visits for influenza-like illness (ILI) was above national baseline levels, while the proportion of outpatient visits for acute respiratory illness (ARI) was below national baseline levels. ILI was below the region specific baselines in the West South Central and Pacific regions, but remained above region-specific baselines in the remaining seven regions. The proportion of outpatient visits reported for ARI was below all region-specific baselines.
• Seventeen states reported widespread influenza activity; 30 states reported regional influenza activity; two states and the District of Columbia reported local influenza activity; and one state and Puerto Rico reported sporadic influenza activity.

* Elevated means the % of visits for ILI or ARI is at or above the national or region-specific baseline
† National data is for current week; regional data is for the most recent 3 weeks.
‡ Includes all 50 states and the District of Columbia

Laboratory Surveillance

During week 12, WHO and NREVSS laboratories reported 4,625 specimens tested for influenza viruses, 986 (21.3%) of which were positive, including 27 influenza A (H1) viruses, 156 influenza A (H3) viruses, 363 influenza A viruses that were not subtyped, and 440 influenza B viruses.

Since September 30, 2007, WHO and NREVSS laboratories have tested a total of 171,992 specimens for influenza viruses and 31,722 (18.4%) were positive. Among the 31,722 influenza viruses, 24,159 (76.2%) were influenza A viruses and 7,563 (23.8%) were influenza B viruses. Seven thousand ninety-eight (29.4%) of the 24,159 influenza A viruses have been subtyped: 1,999 (28.2%) were influenza A (H1) viruses and 5,099 (71.8%) were influenza A (H3) viruses.

Although influenza A (H1) viruses predominated through mid-January, influenza A (H3) viruses have been reported more frequently than influenza A (H1) viruses since week 4 (January 20-26), and during week 4 influenza A (H3) became the predominant virus for the season overall. This season influenza A (H3) viruses have been reported more frequently than A (H1) viruses nationally, as well as in seven of the nine surveillance regions (East North Central, East South Central, Mid-Atlantic, New England, South Atlantic, West North Central, and West South Central). Influenza A (H1) viruses have predominated circulation this season in the remaining two regions (Mountain and Pacific).

Composition of the 2008-09 Influenza Vaccine:

WHO and FDA have recommended that the 2008-09 trivalent influenza vaccine for the Northern Hemisphere contain A/Brisbane/59/2007-like (H1N1), A/Brisbane/10/2007-like (H3N2), and B/Florida/4/2006-like viruses. All three components have been changed from the 2007-08 Northern Hemisphere vaccine formulation. A/Brisbane/10/2007-like (H3N2) and B/Florida/4/2006-like viruses are currently included in the 2008 Southern Hemisphere vaccines. This recommendation was based on surveillance data related to epidemiology and antigenic characteristics, serological responses to 2007-08 vaccines, and the availability of candidate strains and reagents.

Antigenic Characterization:

CDC has antigenically characterized 540 influenza viruses [256 influenza A (H1N1), 127 influenza A (H3N2), and 157 influenza B viruses] collected by U.S. laboratories since September 30, 2007.

Influenza A (H1) [256]

• One hundred seventy-five (69%) of the 256 viruses were characterized as A/Solomon Islands/3/2006-like, the influenza A (H1N1) component of the 2007-08 influenza vaccine for the Northern Hemisphere and the 2008 influenza A (H1N1) component for the Southern Hemisphere.
• Nineteen (7%) of the 256 viruses showed somewhat reduced titers with antisera produced against A/Solomon Islands/3/2006.
• Sixty-two (24%) of the 256 viruses were characterized as A/Brisbane/59/2007-like. A/Brisbane/59/2007 is a recent genetic/antigenic variant which evolved from A/Solomon Islands/03/2006. An A/Brisbane/59/2007-like virus is the WHO recommended strain for the 2008-09 Northern Hemisphere vaccine formulation.

Influenza A (H3) [127]

• Twenty-one (17%) of the 127 viruses were characterized as A/Wisconsin/67/2005-like, the influenza A (H3N2) component of the 2007-08 influenza vaccine for the Northern Hemisphere.
• Ninety-seven (76%) of the 127 viruses were characterized as A/Brisbane/10/2007-like. A/Brisbane/10/2007-like viruses are a recent antigenic variant which evolved from, but are antigenically distinct from, A/Wisconsin/67/2005-like viruses. A/Brisbane/10/2007-like virus is the recommended influenza A (H3N2) component for the 2008 Southern Hemisphere and 2008-09 Northern Hemisphere vaccines.
• Nine (7%) of the 127 viruses showed somewhat reduced titers with antisera produced against A/Wisconsin/67/2005 and A/Brisbane/10/2007.

Influenza B (B/Victoria/02/87 and B/Yamagata/16/88 lineages) [157]

Victoria lineage [8]  
• Eight (5%) of the 157 influenza B viruses characterized belong to the B/Victoria lineage of viruses.
  o Six (75%) of these 8 viruses were characterized as B/Ohio/01/2005-like. The recommended influenza B component for the 2007-08 influenza vaccine is a B/Malaysia/2506/2004-like virus, belonging to the B/Victoria lineage. B/Ohio/01/2005 is a recent B/Malaysia/2506/2004-like reference strain.
  o Two (25%) of these 8 viruses showed somewhat reduced titers with antisera produced against B/Ohio/01/2005 and B/Malaysia/2506/2004.

Yamagata lineage [149]  
• One hundred forty-nine (95%) of the 157 influenza B viruses characterized belong to the B/Yamagata lineage of viruses.
  o One hundred forty-eight (99%) of these 149 viruses were identified as B/Florida/04/2006-like, the recommended influenza B component for the 2008-09 Northern Hemisphere vaccine formulation.
  o One (1%) of these 149 viruses showed a somewhat reduced titer with antiserum produced against B/Florida/04/2006.

These data indicate similarities and differences between a sample of circulating strains and this year’s vaccine strains as determined by laboratory studies. Clinical vaccine effectiveness cannot be accurately predicted using these data, and in previous years, influenza vaccination has been shown to provide measurable protection against influenza illness and influenza-related complications, even when vaccine strains are antigenically distinct from circulating strains.

Antiviral Resistance:

In the United States, two groups of antiviral drugs have been approved by FDA for use in treating or preventing influenza virus infections. These two groups of antiviral drugs are: neuraminidase inhibitors (oseltamivir and zanamivir) and adamantanes (amantadine and rimantadine). A description of these drugs can be found at:http://www.cdc.gov/flu/protect/antiviral/index.htm.

Neuraminidase Inhibitor Antiviral Drugs: So far this season, 1,045 influenza A and B viruses from the United States have been tested for antiviral resistance. Seventy-one (7.8%) of 910 influenza A viruses tested, and 0 (0.0%) of 135 influenza B viruses tested have been found to be resistant to oseltamivir. Currently all of the resistant viruses are H1N1 viruses, with 71 (9.5%) of 746 H1N1 viruses tested exhibiting a genetic mutation that confers oseltamivir resistance. All tested viruses retain their sensitivity to zanamivir. Additional information on antiviral resistance can be found at: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5606a1.htm

Adamantane Antiviral Drugs: Resistance to the adamantanes continues to be high among influenza A (H3N2) viruses with 154 (99.4%) of 155 influenza A (H3N2) viruses tested resistant to the adamantanes. Adamantane resistance among influenza A (H1N1) viruses has also been detected but at a lower level. Of 660 influenza A (H1N1) viruses tested, 80 (12.1%) were resistant to the adamantanes. Since late January, influenza A (H3N2) viruses have predominated in the United States, and during week 12, 85.2% of influenza A viruses subtyped were A (H3N2). The adamantanes are not effective against influenza B viruses.

Based on the level of oseltamivir resistance observed in only one influenza subtype, H1N1, persisting high levels of resistance to the adamantanes in H3N2 viruses, and the predominance of H3N2 viruses circulating in the United States during the 2007-08 season with co-circulation of influenza B viruses, CDC continues to recommend the use of oseltamivir and zanamivir for the treatment or prevention of influenza. Use of amantadine or rimantadine is not recommended. Guidance on influenza antiviral use can be found at: http://www.cdc.gov/mmwr/preview/mmwrhtml/rr5606a1.htm

Pneumonia and Influenza (P&I) Mortality Surveillance

During week 12, 8.9% of all deaths reported through the 122 Cities Mortality Reporting System were reported as due to P&I. This percentage is above the epidemic threshold of 7.1% for week 12. Including week 12, P&I mortality has been above epidemic threshold for eleven consecutive weeks.

Influenza-Associated Pediatric Mortality

Five influenza-associated pediatric deaths were reported to CDC during week 12 (CA, MA[2], NY, and PA). These deaths occurred between February 18 and March 16, 2008. Since September 30, 2007, CDC has received a total of 53 reports of influenza-associated pediatric deaths that occurred during the current season.

Influenza-Associated Pediatric Hospitalizations

Laboratory-confirmed influenza-associated pediatric hospitalizations are monitored in two population-based surveillance networks: the New Vaccine Surveillance Network (NVSN) and the Emerging Infections Program (EIP). These two systems provide updates of surveillance data every two weeks. As a result of differing dates for initiating surveillance in the 2007-08 season, these updates occur on alternating weeks.

During November 4, 2007-March 8, 2008, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the NVSN for children 0-4 years old was 4.95 per 10,000.

During September 30 – March 15, 2008, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the EIP for children 0–17 years old was 1.14 per 10,000. For children aged 0-4 years and 5-17 years, the rate was 2.98 per 10,000 and 0.38 per 10,000, respectively.

Nationwide during week 12, 2.6% of outpatient visits reported through the U.S. Influenza Sentinel Provider Surveillance Network were due to influenza-like illness (ILI), which is above the national baseline of 2.2%. On a regional level, the percentage of visits for ILI decreased in eight of the nine regions compared to week 11 and ranged from 1.6% to 4.0%. Seven of the nine regions reported ILI above their region-specific baselines (East North Central, East South Central, Mid-Atlantic, Mountain, New England, South Atlantic, and West North Central), and two of the nine regions (Pacific and West South Central) reported ILI below their region specific baselines.

During week 12, 2.5% of patient visits to Department of Veteran’s Affairs (VA) and Department of Defense (DoD) outpatient treatment facilities were for acute respiratory illness (ARI), which was below the national baseline of 3.2%. On a regional level, the percentage of visits for ARI ranged from 1.4% to 3.0%, and was below region-specific baselines in all nine regions. All five age groups reported ARI below their age-specific baselines.

Geographic Spread of Influenza as Assessed by State and Territorial Epidemiologists

During week 12 the following influenza activity was reported:

• Widespread activity was reported by 17 states (Alaska, Arizona, Colorado, Connecticut, Delaware, Georgia, Illinois, Iowa, Maine, Nebraska, New York, North Carolina, Pennsylvania, Rhode Island, South Dakota, Vermont, and Virginia).
• Regional activity was reported by 30 states (Alabama, California, Florida, Hawaii, Idaho, Indiana, Kansas, Kentucky, Louisiana, Maryland, Michigan, Minnesota, Missouri, Montana, Nevada, New Hampshire, New Jersey, New Mexico, North Dakota, Ohio, Oklahoma, Oregon, South Carolina, Tennessee, Texas, Utah, Washington, West Virginia, Wisconsin, and Wyoming).
• Local influenza activity was reported by the District of Columbia and 2 states (Massachusetts and Mississippi).
• Sporadic influenza activity was reported by Puerto Rico and one state (Arkansas)