Update: Influenza Activity --- United States, January 15--21, 2006

During January 15--21, 2006,* the number of states reporting widespread influenza activity^† decreased to five. Twenty-three states reported regional activity, nine reported local activity, and 13 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 (919 positives) and Pacific (684 positives) regions, accounting for 30.6% and 22.8%, 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 January 21 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 January 21.

Laboratory Surveillance

During January 15--21, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States reported testing 2,283 specimens for influenza viruses, of which 247 (10.8%) were positive. Of these, 81 were influenza A (H3N2) viruses, 159 were influenza A viruses that were not subtyped, and seven were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested 50,688 specimens for influenza viruses, of which 3,000 (5.9%) were positive. Of these, 2,904 (96.8%) were influenza A viruses, and 96 (3.2%) were influenza B viruses. Of the 2,904 influenza A viruses, 1,388 (47.8%) have been subtyped; 1,381 (99.5%) were influenza A (H3N2) viruses, and seven (0.5%) were influenza A (H1N1) viruses.

P&I Mortality and ILI Surveillance

During the week ending January 21, 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 0.9% in the New England region to 6.0% in the West South Central region.

Pediatric Deaths and Hospitalizations

During October 2, 2005--January 21, 2006, CDC received reports of 11 influenza-associated deaths in U.S. residents aged <18 years. Nine of the deaths occurred during the current influenza season, and two occurred during the 2004--05 influenza season.

During October 1, 2005--January 7, 2006, the preliminary influenza-associated hospitalization rate reported by the Emerging Infections Program^§§ (EIP) for children aged 0--17 years was 0.18 per 10,000. For children aged 0--4 years and 5--17 years, the rate was 0.48 per 10,000 and 0.02 per 10,000, respectively. During October 30, 2005--January 7, 2006, the New Vaccine Surveillance Network^¶¶ (NVSN) reported no laboratory-confirmed influenza-associated hospitalizations among children aged 0--4 years. EIP and NVSN hospitalization rate estimates are preliminary.

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 January 30, 2006, a total of 160 laboratory-confirmed human avian influenza A (H5N1) infections were reported to WHO from Cambodia, China, Indonesia, Thailand, Turkey, and Viet Nam.*** Of these, 85 (53%) were fatal (Table). This represents an increase of one case and one death in China and eight cases and two deaths in Turkey since January 23, 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).

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 27. 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: Colorado, Connecticut, Kansas, Texas, and Wyoming; regional: Alaska, Arizona, Arkansas, California, Georgia, Idaho, Iowa, Kentucky, Maryland, Minnesota, Mississippi, Nebraska, New Mexico, New York, North Carolina, Oregon, Pennsylvania, Rhode Island, Tennessee, Utah, Virginia, Washington, and Wisconsin; local: Florida, Indiana, Massachusetts, Michigan, Montana, Nevada, Ohio, Oklahoma, and South Dakota; sporadic: Alabama, Delaware, Hawaii, Illinois, Louisiana, Maine, Missouri, New Hampshire, New Jersey, North Dakota, South Carolina, Vermont, and West Virginia; 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 (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.

^¶¶ The New Vaccine Surveillance Network (NVSN) 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: 2/2/2006