Update: Influenza Activity --- United States, January 22--28, 2006

During January 22--28, 2006,* the number of states reporting widespread influenza activity^† remained at five. Twenty-one states reported regional activity, 13 reported local activity, and 11 reported sporadic activity (Figure 1).^§

The percentage of specimens testing positive for influenza increased in the United States overall. During the past 3 weeks (i.e., weeks 2--4), the largest number of isolates have been reported from the Mountain and West South Central regions. During this time, the percentage of specimens testing positive for influenza has ranged from 20.9% and 20.6% in the East North Central and West South Central regions, respectively, to 4.9% in the East South Central region. The percentage of outpatient visits for influenza-like illness (ILI)^¶ increased during the week ending January 28 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 28.

Laboratory Surveillance

During January 22--28, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System (NREVSS) laboratories in the United States reported testing 2,854 specimens for influenza viruses, of which 343 (12.0%) were positive. Of these, 117 were influenza A (H3N2) viruses, two were influenza A (H1N1) viruses, 212 were influenza A viruses that were not subtyped, and 12 were influenza B viruses.

Since October 2, 2005, WHO and NREVSS laboratories have tested 56,596 specimens for influenza viruses, of which 3,771 (6.7%) were positive. Of these, 3,654 (96.9%) were influenza A viruses, and 117 (3.1%) were influenza B viruses. Of the 3,654 influenza A viruses, 1,802 (49.3%) have been subtyped; 1,787 (99.2%) were influenza A (H3N2) viruses, and 15 (0.8%) were influenza A (H1N1) viruses.

P&I Mortality and ILI Surveillance

During the week ending January 28, P&I accounted for 7.6% of all deaths reported through the 122 Cities Mortality Reporting System. This percentage was below the epidemic threshold^†† of 8.2% (Figure 2).

The percentage of patient visits for ILI was 2.4%, which is above the national baseline of 2.2% (Figure 3). The percentage of patient visits for ILI ranged from 1.4% in the New England region to 5.9% in the West South Central region.

Pediatric Deaths and Hospitalizations

During October 2, 2005--January 28, 2006, CDC received reports of 13 influenza-associated deaths in U.S. residents aged <18 years. Eleven 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 influenza-associated hospitalization rate reported by the Emerging Infections Program^§§ (EIP) 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^¶¶ (NVSN) was 0.21 per 10,000. 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 February 6, 2006, a total of 165 laboratory-confirmed human avian influenza A (H5N1) infections were reported to WHO from Cambodia, China, Indonesia, Iraq, Thailand, Turkey, and Vietnam.*** Of these, 88 (53%) were fatal (Table). This represents an increase of four cases and two deaths in Indonesia since January 30, 2006, and the first case and death reported in Iraq. 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 February 3. 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, Florida, Iowa, Minnesota, Mississippi, Nebraska, New Mexico, New York, North Carolina, Oklahoma, Oregon, Pennsylvania, Rhode Island, South Carolina, Tennessee, Virginia, and Wisconsin; local: Idaho, Illinois, Indiana, Maine, Massachusetts, Michigan, Missouri, Montana, Nevada, North Dakota, Ohio, South Dakota, and Washington; sporadic: Alabama, Delaware, Hawaii, Kentucky, Louisiana, Maryland, New Hampshire, New Jersey, Utah, 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 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 http://www.who.int/csr/disease/avian_influenza/en.

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Date last reviewed: 2/8/2006