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Update: Influenza Activity --- United States, January 29--February 4, 2006

During January 29--February 4, 2006,* the number of states reporting widespread influenza activity increased to nine. Twenty-one states reported regional activity, 13 reported local activity, and six reported sporadic activity (Figure 1).§

The percentage of specimens testing positive for influenza increased in the United States overall. During weeks 3--5, the largest number of isolates were reported from the Mountain and West South Central regions; the percentage of specimens testing positive for influenza ranged from 20.5% in the West South Central region to 3.7% in the East South Central region. The percentage of outpatient visits for influenza-like illness (ILI) decreased during the week ending February 4 but remained above the national baseline.** The percentage of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold for the week ending February 4.

Laboratory Surveillance

During January 29--February 4, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States reported testing 2,401 specimens for influenza viruses, of which 333 (13.9%) were positive. Of these, 96 were influenza A (H3N2) viruses, two were influenza A (H1N1) viruses, 211 were influenza A viruses that were not subtyped, and 24 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested 61,861 specimens for influenza viruses, of which 4,466 (7.2%) were positive. Of these, 4,312 (96.6%) were influenza A viruses, and 154 (3.4%) were influenza B viruses. Of the 4,312 influenza A viruses, 2,069 (48.0%) have been subtyped; 2,048 (99.0%) were influenza A (H3N2) viruses, and 21 (1.0%) were influenza A (H1N1) viruses.

P&I Mortality and ILI Surveillance

During the week ending February 4, P&I accounted for 7.4% of all deaths reported through the 122 Cities Mortality Reporting System. This percentage is below the epidemic threshold†† of 8.2% (Figure 2).

The percentage of patient visits for ILI was 2.3%, which is above the national baseline of 2.2% (Figure 3). The percentage of patient visits for ILI ranged from 1.4% in the West North Central region to 4.6% in the West South Central region.

Pediatric Deaths and Hospitalizations

During October 2, 2005--February 4, 2006, CDC received reports of 14 influenza-associated deaths in U.S. residents aged <18 years. Twelve of the deaths occurred during the current influenza season, and two occurred during the 2004--05 influenza season.

During October 1, 2005--January 21, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the Emerging Infections Program§§ for children aged 0--17 years was 0.24 per 10,000. For children aged 0--4 years and 5--17 years, the rate was 0.66 per 10,000 and 0.04 per 10,000, respectively. During October 30, 2005--January 21, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0--4 years in the New Vaccine Surveillance Network¶¶ was 0.21 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 February 13, 2006, a total of 169 laboratory-confirmed human avian influenza A (H5N1) infections were reported to WHO from Cambodia, China, Indonesia, Iraq, Thailand, Turkey, and Vietnam.*** Of these, 91 (54%) were fatal (Table). This represents an increase of two cases and one death in China and two cases and two deaths in Indonesia since February 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 February 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: Colorado, Connecticut, Kansas, New York, Pennsylvania, South Carolina, Texas, Virginia, and Wyoming; regional: Alabama, Alaska, Arkansas, California, Florida, Georgia, Indiana, Iowa, Kentucky, Louisiana, Maryland, Massachusetts, Minnesota, Mississippi, Nebraska, North Carolina, Oklahoma, Oregon, Rhode Island, Tennessee, and Wisconsin; local: Arizona, Idaho, Illinois, Maine, Michigan, Montana, Nevada, New Jersey, New Mexico, North Dakota, Ohio, South Dakota, and Washington; sporadic: Delaware, Hawaii, New Hampshire, Utah, Vermont, and West Virginia; no activity: none; no report: Missouri.

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 Emerging Infections Program 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.

¶¶ The New Vaccine Surveillance Network conducts surveillance in Monroe County, New York; Hamilton County, Ohio; and Davidson County, Tennessee.

*** Available at

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Date last reviewed: 2/16/2006


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