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Update: Influenza Activity --- United States, March 19--25, 2006

During March 19--25, 2006,* the number of states reporting widespread influenza activity decreased to 16. Sixteen states reported regional activity, 13 reported local activity, and five reported sporadic activity (Figure 1).§

The percentage of specimens testing positive for influenza remained at the same level as recent weeks in the United States. During the preceding 3 weeks (weeks 10--12), the percentage of specimens testing positive for influenza ranged from 30.6% and 28.6% in the South Atlantic and East South Central regions, respectively, to 9.7% in the Pacific region. During this period, 60.3% of isolates from the Mountain region have been influenza B. Other regions reporting >30.0% of recent isolates as influenza B include the East North Central, West North Central, West South Central, and Pacific regions. The percentage of outpatient visits for influenza-like illness (ILI) during the week ending March 25 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 25.

Laboratory Surveillance

During March 19--25, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States reported testing 3,158 specimens for influenza viruses, of which 681 (21.6%) were positive. Of these, 77 were influenza A (H3N2) viruses, 70 were influenza A (H1N1) viruses, 347 were influenza A viruses that were not subtyped, and 187 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested 110,066 specimens for influenza viruses, of which 13,532 (12.3%) were positive. Of these, 11,989 (88.6%) were influenza A viruses, and 1,543 (11.4%) were influenza B viruses. Of the 11,989 influenza A viruses, 4,875 (40.7%) have been subtyped; 4,629 (95.0%) were influenza A (H3N2) viruses, and 246 (5.0%) were influenza A (H1N1) viruses.

P&I Mortality and ILI Surveillance

During the week ending March 25, P&I accounted for 7.7% 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 2.5%, which is above the national baseline of 2.2% (Figure 3). The percentage of patient visits for ILI ranged from 1.4% in the East South Central region to 3.5% in the West South Central region.

Pediatric Deaths and Hospitalizations

During October 2, 2005--March 25, 2006, CDC received reports of 19 influenza-associated deaths in U.S. residents aged <18 years. Sixteen of the deaths occurred during the current influenza season, and three occurred during the 2004--05 influenza season.

During October 1, 2005--March 18, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate reported by the Emerging Infections Program§§ for children aged 0--17 years was 0.79 per 10,000. For children aged 0--4 years and 5--17 years, the rate was 1.88 per 10,000 and 0.22 per 10,000, respectively. During October 30, 2005--March 18, 2006, the preliminary laboratory-confirmed influenza-associated hospitalization rate for children aged 0--4 years in the New Vaccine Surveillance Network¶¶ was 3.0 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 April 4, 2006, a total of 191 laboratory-confirmed human avian influenza A (H5N1) infections were reported to WHO from Azerbaijan, Cambodia, China, Egypt, Indonesia, Iraq, Thailand, Turkey, and Vietnam.*** Of these, 108 (57%) were fatal (Table). This represents an increase of one case and one death in Indonesia and four cases and two deaths in Egypt since March 24 and the first human infections with avian influenza A (H5N1) reported in Egypt. 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 24. 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: Arkansas, Connecticut, Delaware, Indiana, Kentucky, Maine, Maryland, Massachusetts, New York, North Dakota, Ohio, Rhode Island, South Carolina, Vermont, Virginia, and West Virginia; regional: Georgia, Hawaii, Illinois, Iowa, Michigan, Minnesota, Montana, Nebraska, New Hampshire, New Jersey, North Carolina, Pennsylvania, South Dakota, Tennessee, Texas, and Wisconsin; local: Alabama, Alaska, California, Colorado, Florida, Idaho, Kansas, Missouri, Nevada, Oklahoma, Oregon, Washington, and Wyoming; sporadic: Arizona, Louisiana, Mississippi, New Mexico, and Utah; no activity: none; no report: none.

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: 4/6/2006


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