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NOTE: This document is provided for historical purposes only and may not reflect the most accurate and up-to-date information on this subject. For current flu information, please visit the CDC Flu Homepage.


During the 2004-05 U.S. season, influenza activity occurred at low levels from October to mid-December, steadily increased during January and peaked in mid-February. Influenza A (H3N2) viruses predominated but influenza A (H1)† and B viruses also circulated. This summary is based on data reported by U.S. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) Collaborating Laboratories, U.S. Influenza Sentinel Providers, U.S. State and Territorial Epidemiologists, the 122 Cities Mortality Reporting System, the Emerging Infections Program (EIP), the New Vaccine Surveillance Network (NVSN), and the Nationally Notifiable Disease Surveillance System (NNDSS).

Outbreaks of avian influenza A (H5N1) among poultry continued to be reported in Southeast Asia throughout the 2004-05 season. From mid-December 2004 to June 28, 2005 these outbreaks were associated with human infections and deaths in Vietnam (60 cases and 18 deaths) and Cambodia (4 cases, all fatal). In February 2004, CDC issued recommendations for enhanced surveillance for human infection with avian influenza among travelers returning to the United States from H5N1 affected countries. As of July 6, these recommendations remain in effect and can be found on the CDC Web site at: http://www2a.cdc.gov/han/ArchiveSys/ViewMsgV.asp?AlertNum=00204.

Laboratory Surveillance*:

Since October 3, 2004, WHO and NREVSS laboratories tested 157,759 specimens for influenza viruses, of which 23,549 (14.9%) were positive. Of these, 17,750 (75.4%) were influenza A viruses, and 5,799 (24.6%) were influenza B viruses. Of the 17,750 influenza A viruses, 5,819 (32.8%) have been subtyped; 5,801 (99.7%) were influenza A (H3N2) viruses and 18 (0.3%) were influenza A (H1) viruses.

INFLUENZA Virus Isolated

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Nationwide, the percentage of respiratory specimens testing positive for influenza viruses peaked at 27.0% during the week ending February 5, 2005 (week 5); however, the largest number of isolates was reported during the week ending February 19 (week 7). The peak percentage of specimens testing positive for influenza viruses during the previous 4 seasons (2000-01, 2001-02, 2002-03, and 2003-04) ranged from 25% to 35%. Within the nine surveillance regions, the timing of peak percentages positive during the 2004-05 season ranged from the week ending January 8 (week 1) in the New England region to the week ending March 5 (week 9) in the South Atlantic region. Appendix 1 contains region-specific data.

Antigenic Characterization§:

CDC has antigenically characterized 1,075 influenza viruses collected by U.S. laboratories since October 1, 2004: 11 influenza A(H1N1) viruses, 709 influenza A(H3N2) viruses, and 355 influenza B viruses. The hemagglutinin proteins of the influenza A(H1N1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99. One hundred fifty-six (22%) of the 709 influenza A(H3N2) isolates were characterized as antigenically similar to A/Wyoming/3/2003, which is the A/Fujian/411/2002-like (H3N2) component of the 2004-05 influenza vaccine, and 553 (78%) were characterized as A/California/7/2004-like. A total of 264 (74.4%) of the influenza B viruses characterized in the 2004-05 season belong to the B/Yamagata/16/88 lineage. Of these, 219 (83.0%) were B/Shanghai/361/2002-like, and 45 (17.0%) had reduced titers to ferret antisera produced against B/Shanghai/361/2002. Ninety-one (25.6%) influenza B viruses belong to the B/Victoria/2/87 lineage.

Influenza B viruses currently circulating can be divided into two antigenically and genetically distinct lineages represented by B/Yamagata/16/88 and B/Victoria/2/87 viruses. B/Yamagata lineage viruses circulated widely between 1990 and 2001, during which time B/Victoria-like viruses were not identified outside of Asia. However, between March 2001 and October 2003, B/Victoria-like viruses predominated in many countries, including the United States, and the vaccine strains were changed accordingly. While both Victoria lineage and Yamagata lineage viruses have been reported worldwide during the past year, Yamagata lineage viruses have predominated.

INFLUENZA Virus Isolated

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Composition of the 2005-06 Influenza Vaccine:

The Food and Drug Administration's Vaccines and Related Biological Products Advisory Committee has recommended that the 2005-06 trivalent influenza vaccine for the United States contain A/New Caledonia/20/99-like (H1N1), A/California/7/2004-like (H3N2), and B/Shanghai/361/2002-like viruses. The influenza A (H3N2) vaccine component has been changed for the 2005-06 influenza season. This recommendation was based on antigenic analyses of recently isolated influenza viruses, epidemiologic data, and post-vaccination serologic studies in humans.

Pneumonia and Influenza (P&I) Mortality Surveillance*:

The proportion of deaths in the United States attributed to pneumonia and influenza (P&I) exceeded the epidemic threshold during 10 consecutive weeks (weeks ending February 19-April 23, 2005). The percentage of P&I deaths reached a peak of 8.9% during the week ending March 5, 2005 (week 9) and stayed above the threshold through the week ending April 23, 2005 (week 16). During the previous 4 influenza seasons, the peak percentage of P&I deaths ranged from 8.5% to 10.4%.

Pneumonia And Influenza Mortality

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Influenza-Associated Pediatric Mortality*:

In October 2004, influenza-associated pediatric deaths became nationally notifiable. As of July 6, 39 pediatric deaths had been reported to CDC from 17 states (California, Colorado, Florida, Georgia, Iowa, Maine, Maryland, Massachusetts, Michigan, Mississippi, Nevada, New Jersey, New York, Ohio, Pennsylvania, Vermont, and Virginia) and New York City during the 2004-05 influenza season. All deaths were reported during January-June 2005.

Influenza-Associated Pediatric Hospitalizations:

The 2004-05 season was the first influenza season to integrate laboratory-confirmed pediatric hospitalization rates into the weekly influenza update. Laboratory-confirmed influenza-associated pediatric hospitalizations are monitored in two population-based surveillance networks: the Emerging Infections Program (EIP), since the 2003-04 season, and the New Vaccine Surveillance Network (NVSN), since the 2000-01 season. During October 3, 2004-April 30, 2005‡, the preliminary influenza-associated hospitalization rate for children 0-4 years old reported by NVSN and EIP was 7.0 and 3.2 per 10,000, respectively. EIP also monitors hospitalizations in children aged 5-17 years, and the preliminary influenza-associated hospitalization rate for this age group was 0.6 per 10,000. The overall hospitalization rate reported by EIP for children aged 0-17 years was 1.3 per 10,000. In years 2000-2004, the end-of-season hospitalization rate for NVSN ranged from 3.7 (2002-03) to 12 (2003-04) per 10,000 children. The 2003-04 end of season hospitalization rate for EIP was 8.9 per 10,000 children aged 0-4 years** and 0.8 per 10,000 for children aged 5-17 years. The difference in rates between NVSN and EIP may be due to different case-finding methods and the different populations monitored. For a summary of the methods used in each system, please refer to the surveillance methods in the Flu Activity section of the CDC influenza website.

NVSN laboratory-confirmed influenza-associated hospitalizations for children 0-4 years old
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EIP Influenza Laboratory-Confirmed Cumulative Hospitalization Rates for Children 0-4 years and 5-17 years, 2004-05 and 2003-04
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Influenza-like Illness (ILI) Surveillance from U.S. Sentinel Sites:

Nationally, the percentage of outpatient visits for ILI reported by U.S. sentinel healthcare providers exceeded baseline levels (2.5%)†† during the week ending January 1 (week 52) and for 11 consecutive weeks during the weeks ending January 15-March 26, 2005 (weeks 2-12). ILI peaked at 5.4% during the week ending February 19, 2005 (week 7). During the previous 4 influenza seasons, the peak percentage of patient visits for ILI ranged from 4.0% to 7.6% and peaked during late December to early February§§. Regional₯₯ peak percentages of patient visits for ILI during the 2004-05 influenza season ranged from 2.9% to 8.9% and all occurred in mid-February. The elevation in the percentage of visits due to ILI during the weeks ending December 25 and January 1 may be due to the holidays when the number of routine visits, which contribute to the denominator, is reduced. Appendix 1 contains region-specific data.

Bar Chart for Influenza-like Illness
View Sentinel Providers Regional Charts | View Chart Data | View Full Screen

Influenza Activity as Assessed by State and Territorial Epidemiologists:

On the basis of data from state and territorial epidemiologist reports, influenza activity¶¶ peaked during the week ending February 19 (week 7) when 15 states reported regional influenza activity and 33 reported widespread activity. The peak number of states reporting regional or widespread activity during the previous 4 seasons ranged from 35 to 50. Widespread activity was reported by one or more states for 18 consecutive weeks from the week ending December 11 (week 49) through the week ending April 9 (week 14). Forty-two states reported widespread activity at least once during the season.

U. S. map for Weekly Influenza Activity


Foot notes

Report prepared: July 7, 2005

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