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The 2002-03 influenza season was mild in the United States; influenza A(H1)† and B viruses circulated widely, and the predominant virus varied by region and time of season. On the basis of 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, and the122 Cities Mortality Reporting System, influenza morbidity peaked during early-to-mid February, 2003 and pneumonia and influenza mortality peaked during late February, 2003. Human infections with avian influenza A(H5N1) and A(H7N7) viruses were reported in Hong Kong and the Netherlands, respectively. CDC issued recommendations for enhanced surveillance to increase the likelihood of timely detection of an importation of human infection with a novel influenza virus. These recommendations are still in effect and can be found on the Internet at www.cdc.gov/ncidod/diseases/flu/hanH5N1.htm . To date, no human cases of infection with these viruses have been reported in the U.S.

 Influenza Virus Isolated
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Laboratory Surveillance

From September 29, 2002 through May 17, 2003, WHO and NREVSS laboratories tested 97,649 respiratory specimens for influenza viruses and 10,948 (11.2%) were positive. Of the positive results, 6,180 (56.4%) were influenza type A viruses and 4,768 (43.6%) were influenza type B viruses. Of the 3,170 influenza A viruses subtyped, 2,228 (70.3%) were A(H1) viruses and 942 (29.7%) were A(H3N2) viruses. Influenza A viruses were reported more frequently (range 57.8%-85.9%) than influenza B viruses in the New England, East North Central, Pacific, Mountain, and Mid-Atlantic regions‡, and influenza B viruses were reported more frequently (range: 51.1%-78.1%) than influenza A viruses in the West North Central, West South Central, South Atlantic, and East South Central regions.

 Influenza Virus Isolated
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Nationwide, the percentage of respiratory specimens testing positive for influenza peaked at 24.7% during the week ending February 8, 2003. The peak percentage of specimens testing positive for influenza during the previous 3 seasons (1999-2000, 2000-01, and 2001-02), ranged from 23% to 31%. During the 2002-03 season, influenza activity as reported by WHO/NREVSS laboratories in the East North Central, East South Central, Mid-Atlantic, New England, South Atlantic, West North Central, and West South Central regions peaked during early to mid-February, while activity in the Mountain and Pacific regions peaked during late February to early March. Appendix 1 contains region specific data.

 Influenza Virus Isolated

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Antigenic Characterization of Viral Isolates

CDC has antigenically characterized 699 influenza viruses submitted by U.S. laboratories and collected between September 29, 2002 and May 17, 2003: two hundred and eighty-seven influenza A (H1)† viruses, 143 influenza A (H3N2) viruses, and 269 influenza B viruses. Two hundred and eleven of the influenza A (H1) viruses had the N1 neuraminidase, and 76 had the N2 neuraminidase. The hemagglutinin proteins of all 287 influenza A (H1) viruses were similar antigenically to the hemagglutinin of the vaccine strain A/New Caledonia/20/99 (H1N1). Of the 143 influenza A (H3N2) isolates that have been characterized, 121 (85%) were similar to A/Panama/2007/99, the H3N2 component of the 2002-03 influenza vaccine, and 22 (15%) showed reduced titers to ferret antisera produced against A/Panama/2007/99. Of the 269 influenza B viruses that have been characterized, 268 belonged to the B/Victoria lineage and were similar antigenically to the vaccine strain B/Hong Kong/330/01 and one belonged to the B/Yamagata lineage and was similar to B/Shizuoka/15/01, a B/Sichuan/379/99-like virus.

Influenza Strains Contained in the 2003-04 Vaccine

The trivalent influenza vaccine for the 2003-04 season will include A/New Caledonia/20/99-like (H1N1), A/Moscow/10/99-like (H3N2), and B/Hong Kong/330/2001-like viruses. For the A/Moscow/10/99-like (H3N2) virus, U.S. manufacturers will use the antigenically equivalent A/Panama/2007/99 (H3N2) virus, and for the B/Hong Kong/330/2001-like virus, they will use either B/Hong Kong/330/01 or the antigenically equivalent virus B/Hong Kong/1434/02. These viruses will be used because of their growth properties and because they are representative of currently circulating A (H3N2) and B viruses.

Pneumonia and Influenza (P&I) Mortality

The percentage of P&I deaths in the United States peaked at 8.3% during the week ending March 1 but did not exceed the epidemic threshold§ during the 2002-03 season. During the 1999-00, 2000-01, and 2001-02 seasons, the number of consecutive weeks during which the percentage of deaths attributed to P&I was above the epidemic threshold ranged from 0 to 13 weeks.

Pneumonia And Influenza Mortality

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Influenza Morbidity Reports from U.S. Sentinel Sites

Nationally, influenza morbidity as reported by U.S. sentinel physicians exceeded baseline levels (0-1.9%)** during the week ending December 28, 2002 and each consecutive week during the weeks ending January 25-March 1, 2003. The peak percentage of patient visits for influenza-like illness (ILI)*** was 3.2% during the week ending February 8. During the previous 3 influenza seasons, the peak percentage of patient visits for ILI ranged between 3.2% and 5.7%. Data for the 2002-03 season suggest that influenza activity peaked during early-to-mid February in the East North Central, East South Central, Mid-Atlantic, Mountain, New England, Pacific, and South Atlantic regions. However, influenza activity as reported by sentinel sites in the West North Central and West South Central regions peaked during mid-to-late January. Appendix 1 contains region specific data.

Bar Chart for Influenza-like Illness

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Influenza Activity as Assessed by State and Territorial Epidemiologists**

On the basis of data from state and territorial epidemiologist reports, influenza activity¥ peaked during late February, when 35 states reported regional or widespread influenza activity. The peak number of states reporting regional or widespread activity during the previous 3 years ranged from 38 to 44. During the 2002-03 season, one or more states reported regional influenza activity during 30 consecutive weeks from the week ending October 26, 2002 through the week ending May 17, 2003. Widespread activity was reported by one or more states during the weeks ending December 7-21, 2002 and for all but 1 week during the weeks ending January 18-April 19, 2003.

Usmap for Weekly Influenza Activity

* This summary includes data reported as of July 3, 2003. Reporting is incomplete and numbers may change as more reports are received.

† Includes influenza A(H1N1) and A(H1N2) viruses.

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

§ The expected 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 over the previous 5 years. The epidemic threshold is 1.654 standard deviations above the seasonal baseline.

** The national baseline was calculated as the mean percentage of visits for ILI during noninfluenza weeks plus two standard deviations. Because of wide variability in regional level data, calculating region-specific baselines is not possible and to apply the national baseline to regional level data in not appropriate.

*** Temperature of >100.0 F (>37.8 C) and either cough or sore throat in the absence of a known cause.

¥ Influenza activity is defined as influenza-like illness and/or culture-confirmed influenza. Levels of activity: 1) no activity, 2) sporadic---sporadically occurring ILI or laboratory-confirmed influenza with no outbreaks detected, 3) regional---outbreaks of ILI or laboratory-confirmed influenza in counties with a combined population of < 50% of the state's population, and 4) widespread---outbreaks of ILI or laboratory-confirmed influenza in counties with a combined population of > 50% of the state's population.


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