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Update: Influenza Activity --- United States, February 26--March 4, 2006

During February 26--March 4, 2006,* the number of states reporting widespread influenza activity increased to 25. Thirteen states reported regional activity, eight reported local activity, and four reported sporadic activity (Figure 1).§

The percentage of specimens testing positive for influenza increased in the United States overall. During the preceding 3 weeks (weeks 7--9), the percentage of specimens testing positive for influenza ranged from 29.5% in the East South Central region to 10.8% in the Pacific region. During this period, 36.2% and 26.3% of isolates from the Mountain and West South Central regions, respectively, have been influenza B. Combined, the influenza B isolates reported from these regions account for 66.3% of the B isolates reported during the preceding 3 weeks. The percentage of outpatient visits for influenza-like illness (ILI) decreased during the week ending March 4 but remains above the national baseline.** The percentage of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold for the week ending March 4.

Laboratory Surveillance

During February 26--March 4, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States reported testing 3,239 specimens for influenza viruses, of which 701 (21.6%) were positive. Of these, 175 were influenza A (H3N2) viruses, 20 were influenza A (H1N1) viruses, 399 were influenza A viruses that were not subtyped, and 107 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested 89,513 specimens for influenza viruses, of which 9,143 (10.2%) were positive. Of these, 8,546 (93.5%) were influenza A viruses, and 597 (6.5%) were influenza B viruses. Of the 8,546 influenza A viruses, 3,675 (43.0%) have been subtyped; 3,590 (97.7%) were influenza A (H3N2) viruses, and 85 (2.3%) were influenza A (H1N1) viruses.

P&I Mortality and ILI Surveillance

During the week ending March 4, P&I accounted for 7.0% of all deaths reported through the 122 Cities Mortality Reporting System. This percentage is below the epidemic threshold†† of 8.3% (Figure 2).

The percentage of patient visits for ILI was 3.0%, which is above the national baseline of 2.2% (Figure 3). The percentage of patient visits for ILI ranged from 1.3% in the Pacific region to 4.2% in the West South Central region.

Pediatric Deaths and Hospitalizations

During October 2, 2005--March 4, 2006, CDC received reports of 16 influenza-associated deaths in U.S. residents aged <18 years. Fourteen of the deaths occurred during the current influenza season, and two occurred during the 2004--05 influenza season.

During October 1, 2005--February 18, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the Emerging Infections Program (EIP)§§ for children aged 0--17 years was 0.48 per 10,000. For children aged 0--4 years and 5--17 years, the rate was 1.11 per 10,000 and 0.15 per 10,000, respectively. During October 30, 2005--February 18, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0--4 years in the New Vaccine Surveillance Network (NVSN)¶¶ was 0.48 per 10,000.

Human Avian Influenza A (H5N1)

No human avian influenza A (H5N1) virus infection has ever been identified in the United States. From December 2003 through March 13, 2006, a total of 177 laboratory-confirmed human avian influenza A (H5N1) infections were reported to WHO from Cambodia, China, Indonesia, Iraq, Thailand, Turkey, and Vietnam.*** Of these, 98 (55%) were fatal (Table). This represents an increase of one death in China and two cases and two deaths in Indonesia since March 6, 2006. The majority of infections appear to have been acquired from direct contact with infected poultry. No evidence of sustained human-to-human transmission of H5N1 has been detected, although rare instances of human-to-human transmission likely have occurred (1).


  1. Ungchusak K, Auewarakul P, Dowell SF, et al. Probable person-to-person transmission of avian influenza A (H5N1). N Engl J Med 2005;352:333--40.

* Provisional data reported as of March 10. Additional information about influenza activity is updated each Friday and is available from CDC at

Levels of activity are 1) widespread: outbreaks of influenza or increases in influenza-like illness (ILI) cases and recent laboratory-confirmed influenza in at least half the regions of a state; 2) regional: outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least two but less than half the regions of a state; 3) local: outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of a state; 4) sporadic: small numbers of laboratory-confirmed influenza cases or a single influenza outbreak reported but no increase in cases of ILI; and 5) no activity.

§ Widespread: Alabama, Arkansas, Colorado, Connecticut, Delaware, Florida, Georgia, Indiana, Iowa, Kansas, Kentucky, Louisiana, Maryland, Minnesota, Montana, New York, North Carolina, Ohio, Pennsylvania, Rhode Island, South Carolina, Tennessee, Texas, Vermont, and Virginia; regional: Illinois, Maine, Michigan, Missouri, Nebraska, Nevada, New Hampshire, New Jersey, North Dakota, Oklahoma, South Dakota, Wisconsin, and Wyoming; local: Alaska, California, Hawaii, Idaho, Massachusetts, Mississippi, Oregon, and West Virginia; sporadic: Arizona, New Mexico, Utah, and Washington; no activity: none; no report:

Temperature of >100.0°F (>37.8°C) and cough and/or sore throat in the absence of a known cause other than influenza.

** The national baseline was calculated as the mean percentage of visits for ILI during noninfluenza weeks for the preceding three seasons, plus two standard deviations. Noninfluenza weeks are those in which <10% of laboratory specimens are positive for influenza. Wide variability in regional data precludes calculating region-specific baselines; therefore, applying the national baseline to regional data is inappropriate.

†† The expected seasonal baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected using a robust regression procedure in which a periodic regression model is applied to the observed percentage of deaths from P&I that occurred during the preceding 5 years. The epidemic threshold is 1.645 standard deviations above the seasonal baseline.

§§ The EIP Influenza Project conducts surveillance in 60 counties associated with 12 metropolitan areas: San Francisco, California; Denver, Colorado; New Haven, Connecticut; Atlanta, Georgia; Baltimore, Maryland; Minneapolis/St. Paul, Minnesota; Albuquerque, New Mexico; Las Cruces, New Mexico; Albany, New York; Rochester, New York; Portland, Oregon; and Nashville, Tennessee.

¶¶ NVSN conducts surveillance in Monroe County, New York; Hamilton County, Ohio; and Davidson County, Tennessee.

*** Available at

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Date last reviewed: 3/16/2006


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