Update: Influenza Activity --- United States, December 25--31, 2005

During December 25--31, 2005,* the number of states reporting widespread influenza activity^† increased to seven. Three states reported regional activity, nine reported local activity, and 27 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 (432 positives) and Pacific (302) regions, accounting for 35.9% and 25.1%, 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 31 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 31.

Laboratory Surveillance

During December 25--31, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States reported testing 1,677 specimens for influenza viruses, of which 169 (10.1%) were positive. Of these, 117 were influenza A (H3N2) viruses, two were influenza A (H1N1) viruses, 48 were other influenza A viruses, and two were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested 35,006 specimens for influenza viruses, of which 1,203 (3.4%) were positive. Of these, 1,153 (95.8%) were influenza A viruses, and 50 (4.2%) were influenza B viruses. Of the 1,153 influenza A viruses, 608 (52.7%) have been subtyped; 602 (99.0%) were influenza A (H3N2) viruses, and six (1.0%) were influenza A (H1N1) viruses.

P&I Mortality and ILI Surveillance

During the week ending December 31, P&I accounted for 6.8% of all deaths reported through the 122 Cities Mortality Reporting System. This percentage is below the epidemic threshold^†† of 7.9% (Figure 2).

The percentage of patient visits for ILI was 3.3%, which is above the national baseline of 2.2% (Figure 3). The percentage of patient visits for ILI increased in seven surveillance regions and ranged from 1.6% in the New England region to 6.7% in the West South Central region.

Pediatric Deaths and Hospitalizations

During October 2--December 31, CDC received reports of five influenza-associated deaths of U.S. residents aged <18 years. Three of the deaths occurred during the current influenza season and two occurred during the 2004--05 influenza season.

During October 1--December 24, the preliminary influenza-associated hospitalization rate for children aged 0--4 years reported by the Emerging Infections Program (EIP)^§§ was 0.17 per 10,000 population. EIP also monitors hospitalizations in children aged 5--17 years. The preliminary influenza-associated hospitalization rate for this age group reported by EIP was 0.01 per 10,000 population. During October 30--December 24, the New Vaccine Surveillance Network^¶¶ reported no laboratory-confirmed influenza-associated hospitalizations among children aged 0--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 January 10, 2006, a total of 147 laboratory-confirmed human cases of avian influenza A (H5N1) infections were reported to WHO.*** Of these, 78 (53%) were fatal (Table). Cases were reported from Cambodia, China, Indonesia, Thailand, Turkey, and Viet Nam. Since December 30, four new cases and two deaths in Turkey and one new case and two deaths in China were reported. Cases reported from Turkey are the first human cases reported outside of China or Southeast Asia. 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).

Reference

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 January 6. Additional information about influenza activity is updated each Friday and is available from CDC at http://www.cdc.gov/flu.

^† 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, Colorado, Nevada, New Mexico, Texas, and Utah; regional: Idaho, Kansas, and Oregon; local: Connecticut, Delaware, Mississippi, Montana, Nebraska, Ohio, Oklahoma, Pennsylvania, and Washington; sporadic: Alabama, Alaska, Florida, Georgia, Hawaii, Illinois, Indiana, Iowa, Kentucky, Maine, Massachusetts, Michigan, Minnesota, Missouri, New Hampshire, New Jersey, New York, North Carolina, North Dakota, Rhode Island, South Carolina, South Dakota, Tennessee, Virginia, West Virginia, Wisconsin, and Wyoming; no activity: Arkansas and Louisiana; no report: Maryland and Vermont.

^¶ 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 the following 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 http://www.who.int/csr/disease/avian_influenza/en.

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Date last reviewed: 1/12/2006