Update: Influenza Activity --- United States, January 18--24, 2004

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The number of states reporting widespread influenza activity* continued to decrease during the reporting week of January 18--24, 2004. One state health department reported widespread activity. A total of 20 states reported regional activity, 19 states and New York City reported local activity, and sporadic activity was reported by nine states, the District of Columbia, Guam, and Puerto Rico. The percentage of outpatient visits for influenza-like illness (ILI)^§ remained the same during the week ending January 24. The percentage of specimens testing positive for influenza and the percentage of deaths attributed to pneumonia and influenza (P&I) decreased.

Laboratory Surveillance

During the week ending January 24, World Health Organization (WHO) laboratories reported testing 1,136 specimens for influenza viruses, of which 85 (7.5%) were positive. Of these, 23 were influenza A (H3N2) viruses, and 62 were influenza A viruses that were not subtyped.

Since September 28, 2003, WHO and National Respiratory and Enteric Virus Surveillance System laboratories have tested 83,218 specimens for influenza viruses, of which 21,599 (26.0%) were positive. Of these, 21,471 (99.4%) were influenza A viruses, and 128 (0.6%) were influenza B viruses. Of the 21,471 influenza A viruses, 5,320 (24.8%) have been subtyped; 5,319 (99.9%) were influenza A (H3N2) viruses, and one (0.1%) was an influenza A (H1) virus.

Antigenic Characterization

Of the 573 influenza viruses collected by U.S. laboratories since October 1, 2003, and characterized antigenically by CDC, 565 were influenza A (H3N2) viruses, two were influenza A (H1) viruses, and six were influenza B viruses. The hemagglutinin proteins of the influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. Of the 565 influenza A (H3N2) isolates that have been characterized, 106 (18.8%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 459 (81.2%) were similar to a drift variant, A/Fujian/411/2002 (H3N2)^¶. Five influenza B viruses characterized were similar antigenically to B/Sichuan/379/99, and one was similar antigenically to B/Hong Kong/330/2001.

P&I Mortality Surveillance

During the week ending January 24, 2004, P&I accounted for 9.7% of all deaths reported through the 122 Cities Mortality Reporting System. P&I mortality appears to have peaked but remains above the epidemic threshold** of 8.2% (Figure 1).

ILI Surveillance

The percentage of patient visits to approximately 1,000 U.S. sentinel providers nationwide for ILI remained at 2.0% for the week ending January 24, which is below the national baseline^§§ of 2.5% (Figure 2). The percentage of patient visits for ILI ranged from 3.1% in the South Atlantic region^¶¶ to 1.0% in the West North Central region.

Activity Reported by State and Territorial Epidemiologists

During the week ending January 24, Delaware reported widespread influenza activity. Regional activity was reported in 20 states (Colorado, Connecticut, Florida, Georgia, Idaho, Iowa, Kentucky, Louisiana, Massachusetts, Minnesota, Mississippi, Montana, New Jersey, New York, Ohio, Pennsylvania, Rhode Island, South Carolina, West Virginia, and Wyoming). Local activity was reported in 19 states (Alaska, Arizona, California, Hawaii, Illinois, Indiana, Kansas, Maine, Maryland, Michigan, Missouri, Nebraska, New Hampshire, Oregon, Tennessee, Vermont, Virginia, Washington, and Wisconsin) and New York City. Sporadic activity was reported in nine states (Alabama, Arkansas, Nevada, New Mexico, North Dakota, Oklahoma, South Dakota, Texas, and Utah), the District of Columbia, Guam, and Puerto Rico. North Carolina did not report.

Influenza-Associated Deaths in Children Aged <18 Years

As of January 26, CDC had received reports of 121 influenza-associated deaths in U.S. residents aged <18 years. These data are preliminary and subject to change as more data become available. Thirteen of the 121 deaths occurred since January 1 (Figure 3). All patients had evidence of influenza virus infection detected by rapid-antigen testing or other laboratory tests. Among reported deaths, 62 (51.2%) were male. The median age was 3.8 years (range: 2 weeks--17 years). Of 72 children aged <5 years, 33 were aged 6--23 months. Twenty-six children had medical conditions that put them at increased risk for complications from influenza. Of children whose influenza vaccination status was reported, two were vaccinated according to current recommendations (1), and 57 were not vaccinated.

Weekly influenza activity updates are available through CDC's voice (telephone, 888-232-3228) and fax (telephone, 888-232-3299, document number 361100) information systems. Additional information about influenza viruses and surveillance is available from CDC at http://www.cdc.gov/flu.

Reference

* Levels of activity are 1) no activity, 2) sporadic---small numbers of laboratory-confirmed influenza cases or a single influenza outbreak reported but no increase in cases of influenza-like illness (ILI), 3) local---outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in a single region of a state, 4) 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, and 5) widespread---outbreaks of influenza or increases in ILI cases and recent laboratory-confirmed influenza in at least half the regions of a state.

^§Temperature of >100.0^ºF (>37.8^º C) and cough and/or sore throat in the absence of a known cause other than influenza.

^¶Although vaccine effectiveness against A/Fujian/411/2002-like viruses might be less than that against A/Panama/2007/99-like viruses, the current U.S. vaccine probably will offer some cross-protective immunity against the A/Fujian/411/2002-like viruses and reduce the severity of disease.

** The expected baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected by using a robust regression procedure that applies a periodic regression model to the observed percentage of deaths from P&I during the preceding 5 years; the epidemic threshold is 1.645 standard deviations above the seasonal baseline percentage.

National and regional percentage of patient visits for ILI are weighted on the basis of state population.

^§§Calculated as the mean percentage of visits for ILI during noninfluenza weeks, plus two standard deviations. Wide variability in regional data precludes calculating region-specific baselines and makes it inappropriate to apply the national baseline to regional data.

^¶¶New England=Connecticut, Maine, Massachusetts, New Hampshire, Rhode Island, and Vermont; Mid-Atlantic=New Jersey, New York City, Pennsylvania, and Upstate New York; East North Central=Illinois, Indiana, Michigan, Ohio, and Wisconsin; West North Central=Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, and South Dakota; South Atlantic=Delaware, District of Columbia, Florida, Georgia, Maryland, North Carolina, South Carolina, Virginia, and West Virginia; East South Central=Alabama, Kentucky, Mississippi, and Tennessee; West South Central=Arkansas, Louisiana, Oklahoma, and Texas; Mountain=Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming; and Pacific=Alaska, California, Hawaii, Oregon, and Washington.

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