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Update: Influenza Activity --- United States, December 18--24, 2005

During December 18--24, 2005,* the number of states reporting widespread influenza activity increased to four. Four states reported regional activity, five reported local activity, and 31 reported sporadic activity (Figure 1).§

The percentage of specimens testing positive for influenza increased in the United States overall. Since October 2, 2005, the largest numbers of specimens testing positive for influenza have been reported from the Mountain (309 positives) and Pacific (171) regions, accounting for 44.8% and 24.8%, respectively, of positive tests reported during the 2005--06 influenza season. The percentage of outpatient visits for influenza-like illness (ILI) increased during the week ending December 24 and is above the national baseline.** The percentage of deaths attributed to pneumonia and influenza (P&I) was below the epidemic threshold for the week ending December 24.

Laboratory Surveillance

During December 18--24, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States reported testing 1,147 specimens for influenza viruses, of which 155 (13.5%) were positive. Of these, 94 were influenza A (H3N2) viruses, 60 were other influenza A viruses, and one was an influenza B virus.

Since October 2, 2005, WHO and NREVSS laboratories have tested 27,694 specimens for influenza viruses, of which 690 (2.5%) were positive. Of these, 655 (94.9%) were influenza A viruses, and 35 (5.1%) were influenza B viruses. Of the 655 influenza A viruses, 431 (65.8%) have been subtyped; 427 (99.1%) were influenza A (H3N2) viruses, and four (0.9%) were influenza A (H1N1) viruses.

P&I Mortality and ILI Surveillance

During the week ending December 24, 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 7.8% (Figure 2).

The percentage of patient visits for ILI was 3.1%, which is above the national baseline of 2.2% (Figure 3). The percentage of patient visits for ILI increased in eight surveillance regions and ranged from 0.9% in the West North Central region to 6.3% in the West South Central region.

Pediatric Deaths and Hospitalizations

During October 2--December 24, CDC received reports of five influenza-associated deaths in U.S. residents aged <18 years. Two of the deaths occurred during the 2004--05 influenza season.

During October 1--December 10, the preliminary influenza-associated hospitalization rate for children aged <4 years reported by the Emerging Infections Program was 0.07 per 10,000. No influenza-associated hospitalizations were reported for children aged 5--17 years. During October 30--December 10, the New Vaccine Surveillance Network reported no laboratory-confirmed influenza-associated hospitalizations among children aged <4 years.

Human Cases of Avian Influenza A (H5N1)

No human case of avian influenza A (H5N1) virus infection has ever been identified in the United States. From December 2003 through December 30, 2005, a total of 142 laboratory-confirmed human cases of avian influenza A (H5N1) infections were reported to WHO.§§ Of these, 74 (52%) were fatal (Table). All cases were reported from five countries in Asia (Cambodia, China, Indonesia, Thailand, and Viet Nam). The majority of cases 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 cases 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 December 30. 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: Arizona, California, New Mexico, and Utah; regional: Kansas, Nevada, Oregon, and Texas; local: Colorado, Idaho, Nebraska, Ohio, and Washington; sporadic: Alaska, Connecticut, Delaware, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kentucky, Maine, Maryland, Massachusetts, Michigan, Minnesota, Mississippi, Missouri, Montana, New Jersey, New York, North Carolina, North Dakota, Oklahoma, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia, West Virginia, Wisconsin, and Wyoming ; no activity: Alabama, Arkansas, Louisiana, New Hampshire, and Vermont; no report: South Dakota.

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.

§§ Available at

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Table 4
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Date last reviewed: 1/5/2006


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