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Update: Influenza Activity --- United States, 2003--04 Season

This report summarizes influenza activity in the United States during September 29, 2003--March 27, 2004,* and updates the previous summary (1). This report also summarizes human infections with avian influenza viruses related to poultry outbreaks in North America. Preliminary data collected through CDC influenza surveillance indicate that national influenza activity peaked during late November--December. The most frequently isolated viruses were influenza A (H3N2), and approximately 87% of these were similar to the drift variant A/Fujian/411/2002.

Laboratory Surveillance

As of the week ending March 27, the World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System collaborating laboratories had tested (i.e., by using viral culture and reverse transcriptase--polymerase chain reaction) 115,222 specimens for influenza viruses, of which 24,177 (21.0%) were positive. The percentage of specimens testing positive for influenza viruses exceeded 10.0% during the week ending October 25, 2003, and peaked at 35.2% during the week ending November 29. During the four most recent influenza seasons (1999--00, 2000--01, 2001--02, and 2002--03), the peak percentage of specimens testing positive for influenza viruses ranged from 23.9% to 30.9% (2) (CDC, unpublished data, 2003). Of the 24,177 influenza viruses reported during the 2003--04 season, 23,993 (99.2%) were influenza type A, and 184 (0.8%) were influenza type B viruses. Of the 6,875 influenza type A viruses that were subtyped, 6,873 (99.9%) were influenza A (H3N2) viruses, and two (0.1%) were influenza A (H1) viruses.

Antigenic Characterization

CDC antigenically characterized 863 influenza viruses submitted by U.S. laboratories since October 1, 2003, as follows: 833 influenza A (H3N2) viruses, three influenza A (H1) viruses, and 27 influenza B viruses. Of the 833 influenza A (H3N2) isolates that were characterized, 106 (12.7%) were similar antigenically to the vaccine strain A/Panama/2007/99 (H3N2), and 727 (87.3%) were similar to the drift variant A/Fujian/411/2002 (H3N2). The hemagglutinin proteins of the influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99 (H1N1). Twenty-four of the influenza B viruses belonged to the B/Yamagata lineage and were similar antigenically to B/Sichuan/379/99. Three influenza B viruses belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong.

Influenza-Like Illness Surveillance

During the weeks ending October 4, 2003--March 27, 2004, the weekly percentages of patient visits for influenza-like illness (ILI)§ to approximately 1,000 sentinel providers ranged from 1.0% to 7.6% and exceeded the national baseline of 2.5% for 9 consecutive weeks, from the week ending November 15, 2003, through the week ending January 10, 2004. The peak percentage (7.6%) of patient visits for ILI occurred during the week ending December 27. For the week ending March 27, the percentage of patient visits for ILI was 0.9%.

Activity Reported by State and Territorial Epidemiologists

Regional influenza activity** was reported by state and territorial epidemiologists in at least one state each week during the weeks ending October 11, 2003--February 21, 2004, and widespread activity was first reported for the week ending October 18. During the week ending December 20, widespread influenza activity was reported in 45 states, and regional activity was reported in four additional states. All states reported either regional (eight states) or widespread (42 states) activity during the week ending December 27. Widespread activity was last reported in one state during the week ending January 24, and the last report of regional activity occurred in one state during the week ending February 21. No widespread, regional, or local influenza activity was reported during the week ending March 27.

Pneumonia and Influenza Mortality Surveillance

During the week ending March 27, 2004, an estimated 7.4% of the deaths reported through the 122 Cities Mortality Reporting System were attributed to pneumonia and influenza (P&I). This percentage was below the epidemic threshold†† of 8.2% for that week. For 9 consecutive weeks (weeks ending December 20, 2003, through February 14, 2004), the percentage of P&I deaths exceeded the epidemic threshold; the percentage reached a peak of 10.3% during the week ending January 10.

Influenza-Associated Deaths in Children Aged <18 Years

As of March 27, 2004, CDC had received reports of 142 influenza-associated deaths in U.S. residents aged <18 years occurring in the current season (3). This number represents 21 additional deaths reported since the previous update (1). All patients had evidence of influenza virus infection detected by rapid-antigen testing or other laboratory tests. These data are preliminary and subject to change as more information becomes available.

Avian Influenza Outbreaks

Since early February 2004, avian influenza outbreaks in poultry have been reported from multiple locations in North America, including British Columbia, Delaware, Maryland, New Jersey, Pennsylvania, and Texas (4). Most outbreaks involved influenza A (H7N2) or A (H7N3) strains with low pathogenicity; however, Texas reported an outbreak of highly pathogenic§§ avian influenza A (H5N2) among poultry limited to one farm. The farm was quarantined, depopulated, cleaned, and disinfected. Although no confirmed cases of human infection with avian influenza viruses have occurred to date in relation to these outbreaks in the United States, Canadian health authorities have reported two laboratory-confirmed cases of human influenza A (H7) infection in British Columbia associated with a localized influenza A (H7N3) outbreak in poultry (5,6). Both persons were poultry workers who had separate and known exposure to infected poultry. One person exhibited conjunctivitis and upper respiratory symptoms; the second person had conjunctivitis and headache. Both were treated with oseltamivir and made full recoveries. To date, no human-to-human transmission of H7 influenza A has occurred in Canada.

Reported by: D Skowronski, MD, British Columbia Centre for Disease Control; A King, MD, T Tam, MD, T Wong, MD, Health Canada, Ottawa, Canada. U.S. state and local health departments. Animal and Plant Health Inspection Svc, U.S. Department of Agriculture. L Brammer, MPH, K Teates, MPH, S Harper, MD, A Klimov, PhD, N Cox, PhD, WHO Collaborating Center for Surveillance, Epidemiology and Control of Influenza, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; N Bhat, MD, EIS Officer, CDC.

Editorial Note:

During the 2003--2004 season, influenza activity in the United States appeared earlier than usual (October 2003), peaked during late November--December, and declined rapidly during January--February 2004. Influenza A (H3N2) viruses predominated, with influenza B viruses isolated sporadically. Preliminary data from national influenza surveillance systems indicate that the current season was more severe than the previous three seasons but was within the range expected for a typical A (H3N2) season (2).

Influenza-associated pediatric deaths received considerable attention this season, and CDC requested that state and local health departments report influenza-associated deaths in persons aged <18 years (3). The number of new reported deaths has declined as influenza activity has decreased, with only five new deaths occurring since January 26. Further data collection regarding these reports is ongoing, and efforts are under way to track national pediatric influenza-associated deaths annually.

The avian influenza viruses isolated from the North American poultry outbreaks in 2004 are unrelated to the A (H5N1) epizootic in southeast Asia (4). Influenza A (H7) viruses cause outbreaks among poultry, but do not typically infect humans. In 2002, Virginia experienced an outbreak of avian influenza A (H7N2) in which 4.7 million turkeys and chickens were destroyed. One culler had upper respiratory symptoms and was tested subsequently and found to have antibodies to avian influenza A (H7N2) (7). In 2003, the Netherlands reported outbreaks of avian influenza A (H7N7) in poultry on several farms (8). In that report, a total of 89 persons had confirmed H7N7 influenza virus infection associated with this outbreak, accounting for 83 cases of conjunctivitis, seven cases of ILI, and one death. Since that time, additional H7N7 infections among humans have not been reported. In response to the avian influenza outbreaks in poultry in the United States, CDC has issued interim recommendations for persons with possible exposure to avian influenza. Those recommendations are available at More information regarding human H7 cases in North America is available at

Influenza surveillance reports for the United States are published weekly during October--May. These reports are available at and through CDC's voice (telephone 888-232-3228) and fax (telephone 888-232-3299, document number 361100) information systems.


This report is based on data contributed by participating state and territorial epidemiologists and state public health laboratory directors, World Health Organization collaborating laboratories, National Respiratory and Enteric Virus Surveillance System collaborating laboratories, the U.S. Influenza Sentinel Provider Surveillance System, and Div of Public Health Surveillance and Informatics, Epidemiology Program Office, CDC.


  1. CDC. Update: influenza activity---United States, January 18--24, 2004. MMWR 2004;53:63--5.
  2. CDC. Surveillance for influenza---United States 1997--98, 1998--99, and 1999--2000. In: CDC surveillance summaries (October 25). MMWR 2002;51(No. SS-7).
  3. CDC: Update: influenza-associated deaths reported among children aged <18 years---United States, 2003--04 influenza season. MMWR 2004; 52:1286--8.
  4. U.S. Department of Agriculture. Hot issues. Washington, D.C.: U.S. Department of Agriculture. Animal and Plant Health Inspection Service. Available at
  5. Health Canada. Flu information: avian influenza 2004. Ottawa, Canada: Health Canada. Available at
  6. World Health Organization. Avian influenza A(H7) human infections in Canada. Geneva, Switzerland: World Health Organization. Available at
  7. CDC. Interim report: human infection with avian H7 influenza viruses, North America. Atlanta, Georgia: CDC. Available at
  8. Koopmans M, Wilbrink B, Conyan M, et al. Transmission of H7N7 avian influenza A virus to human beings during a large outbreak in commercial poultry farms in the Netherlands. Lancet 2004;363:587--93.

* As of April 2, 2004. Reporting is incomplete.

Includes both the A (H1N1) and A (H1N2) influenza virus subtypes.

§ Temperature of >100.0º F (>37.8º C) and either cough 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 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.

** Levels of activity are 1) no activity; 2) sporadic---isolated laboratory-confirmed influenza cases or laboratory-confirmed outbreak in one institution, with no increase in activity; 3) local---increased ILI in one region, or at least two institutional outbreaks (ILI or laboratory-confirmed influenza) in one region; virus activity is no greater than sporadic in other regions; 4) regional---increased ILI activity or outbreaks (ILI or laboratory-confirmed influenza) in at least two but fewer than half of the regions in the state, and 5) widespread---increased ILI activity or outbreaks (ILI or laboratory-confirmed influenza) in at least half the regions in the state.

†† The expected seasonal baseline proportion of P&I deaths reported by the 122 Cities Mortality Reporting System is projected by using a robust regression procedure in which a periodic regression model is applied 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 (2).

§§ Avian influenza (AI) viruses are classified into low pathogenic (LPAI) and high pathogenic (HPAI) forms on the basis of genetic sequence and the severity of illness they cause in infected birds. Most AI virus strains are LPAI and typically cause little or no clinical signs in infected birds; however, some LPAI virus strains can mutate under field conditions into HPAI viruses. Additional information is available at

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