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Influenza Activity --- United States, 2000--01 Season

This report summarizes influenza activity in the United States during November 26, 2000--January 13, 2001 (1). Influenza activity was low to moderate but increasing in the United States. Since October 1, the most frequently isolated viruses were influenza A (H1N1) and were well matched antigenically with the 2000--01 influenza A (H1N1) vaccine strain.

During October 1, 2000--January 13, 2001, World Health Organization collaborating laboratories and National Respiratory and Enteric Virus Surveillance System laboratories in the United States tested 26,789 specimens for influenza: 1545 (6%) were positive. Of these, 1132 (73%) were influenza A and 413 (27%) were influenza B. Of the 1132 influenza A isolates collected, 457 (40%) have been subtyped: 441 (96%) were A (H1N1) and 16 (4%) were A (H3N2). Of the 56 influenza A (H1N1) isolates characterized antigenically at CDC, 53 (95%) were A/New Caledonia/20/99-like (H1N1) viruses, the H1N1 component of the 2000--01 vaccine strain, and three (5%) were A/Bayern/07/95-like (H1N1) viruses. The A/New Caledonia/20/99 vaccine strain produces high titers of antibody that are cross-reactive to A/Bayern/07/95-like (H1N1) viruses (2). Ten influenza A (H3N2) viruses and 20 influenza B viruses were characterized; all were similar antigenically to vaccine strains A/Panama/2007/99 (H3N2) and B/Beijing/184/93, respectively. The percentage of positive influenza infections, an important indicator of influenza activity, increased from 4% during the week ending November 25 to 15% during the week ending January 13.

During November 6, 2000--January 13, 2001, 2%--3% of patient visits to U.S. sentinel physicians were for influenza-like illness (ILI)*. During the week ending January 13 (week 2), patient visits for ILI were at baseline levels (0--3%) in seven of nine surveillance regions. Levels were above baseline in the Pacific (6%) and West South Central (4%) regions. During the same week, widespread influenza activity was reported in Rhode Island and Virginia, and regional activity was reported in 21 states (Alabama, Arizona, Colorado, Connecticut, Idaho, Iowa, Kansas, Kentucky, Louisiana, Maryland, Massachusetts, Minnesota, Mississippi, Nevada, North Carolina, Oregon, Tennessee, Texas, Utah, Washington, and Wyoming); 26 states reported sporadic activity, and one state did not report.

The 122 Cities Mortality Reporting System attributed 7.7% of recorded deaths to pneumonia and influenza (P&I). This percentage was below the epidemic threshold^§ of 8.5% for week 2. The percentage of P&I deaths has remained below the epidemic threshold for each week since October 1.

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. WHO collaborating laboratories. National Respiratory and Enteric Virus Surveillance System laboratories. Sentinel Physicians Influenza Surveillance System. Surveillance Systems Br, Div of Public Health Surveillance and Informatics, Epidemiology Program Office; Mortality Statistics Br, Div of Vital Statistics, National Center for Health Statistics; WHO Collaborating Center for Reference and Research on Influenza, Respiratory and Enteric Virus Br, and Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases; and an EIS Officer, CDC.

Editorial Note:

Influenza activity has been low to moderate in the United States; however, the percentage of respiratory specimens that were laboratory-confirmed influenza increased to 15% for the week ending January 13. During peak influenza-activity periods each year from 1990 to 2000, 19%--35% of weekly specimens submitted for respiratory virus testing were positive for influenza viruses. Although influenza A (H1N1) viruses have predominated this season, approximately one fourth of the isolates have been influenza B.

The best prevention against influenza is vaccination; therefore, persons susceptible to complications (3) and close contacts of such persons (e.g., health-care providers and household members who care for high-risk persons) should continue to be vaccinated. An estimated average of 900 deaths and 1300 hospitalizations can be prevented for each additional million elderly persons vaccinated against influenza (CDC, unpublished data, 2000).

Approximately 70.4 million doses of influenza vaccine have been shipped by manufacturers, but another 6.2 million doses of vaccine are available from Aventis Pasteur (Swiftwater, Pennsylvania). This vaccine may be ordered by calling Aventis Pasteur at (800) 822-2463 through February 1, 2001 (4). The minimum order size is five vials (50 doses). Additional information on vaccine prices and ordering procedures is available on the World-Wide Web, http://www.cdc.gov/nip/flu-vac-supply.

CDC collects and reports U.S. influenza surveillance data during October--May. This information is updated weekly and is available through CDC voice information system, telephone (888) 232-3228, the fax information system, telephone (888) 232-3299 (request document no. 361100), or on the World-Wide Web, http://www.cdc.gov/ncidod/diseases/flu/weekly.htm.

References

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

Levels of activity are 1) no activity; 2) sporadic---sporadically occurring ILI or culture-confirmed influenza with no outbreaks detected; 3) regional---outbreaks of ILI or culture-confirmed influenza in counties with a combined population of <50% of the state's population; and 4) widespread---outbreaks of ILI or culture-confirmed influenza in counties with a combined population of >50% of the state's population.

^§ The epidemic threshold is 1.645 standard deviations above the seasonal baseline. The expected seasonal baseline is projected using a robust regression procedure in which a periodic regression model is applied to observed percentages of deaths from P&I since 1983.

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