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

This report summarizes influenza activity in the United States during October 1--November 25, 2000 (1)*. Influenza activity was low during this period, and influenza virus isolates were reported from 11 states. The viruses most frequently isolated were influenza A (H1N1) and were well matched by the 2000--01 influenza vaccine strains.

During October 1--November 25, 1% of patient visits to U.S. sentinel physicians were for influenza-like illness (ILI). During the week ending November 25 (week 47), among each of the nine surveillance regions, patient visits for ILI were at baseline levels (0--3%); 24 state and territorial health departments reported no influenza activity, 24 reported sporadic activity, and two (Kentucky and Texas) reported regional activity (1)§. No states reported widespread activity. The 122 Cities Mortality Reporting System attributed 6.5% of total deaths to pneumonia and influenza (P&I). This percentage was below the epidemic threshold of 7.9% for week 47. Deaths attributed to P&I have remained below the epidemic threshold for each week since October 1 (1).

During October 1--November 25, World Health Organization (WHO) collaborating laboratories and National Respiratory and Enteric Virus Surveillance System laboratories in the United States tested 8511 specimens for influenza; 118 (1.4%) were positive for laboratory-confirmed influenza. Of these, 101 (86%) were influenza A and 17 (14%) were influenza B. The percentage of positive influenza infections identified each week, an important early indicator of influenza activity, increased from zero for the week ending October 21 to 4% for the week ending November 25. Typically, during peak influenza activity, approximately 30%--34% of specimens submitted for respiratory virus testing have tested positive for influenza viruses. Of the 101 influenza A isolates collected, 86 (85%) have been subtyped; 79 (92%) were A (H1N1) and seven (8%) were A (H3N2). Of the three influenza A isolates that were characterized antigenically at CDC, two were A/New Caledonia/20/99-like (H1N1) viruses, the H1N1 component of the 2000--01 vaccine strain, and one was an A/Panama/2007/99-like (H3N2) virus, the H3N2 component of the 2000--01 vaccine strain. One influenza B isolate collected since October 1 was similar to the recommended vaccine strain B/Beijing/184/93.

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, CDC.

Editorial Note: 

All four influenza surveillance system components indicated that influenza activity was low during October--November 25 in the United States, and lower than the same period in 1999. However, the percentage of respiratory specimens that were laboratory-confirmed influenza each week began to increase during this period, and influenza activity is expected to increase during the next few weeks to months. Both influenza A and influenza B viruses were isolated. So far this season, the viruses isolated most frequently were influenza A (H1N1); however, it is too early to know what strain(s) will predominate. Seasonal epidemics caused by influenza A (H1N1) viruses have been less severe than seasons in which influenza A (H3N2) viruses predominated (2). Although a very small number of influenza isolates have been characterized antigenically so far this season, all were well matched to the 2000--01 influenza vaccine strains.

The best prevention against influenza is vaccination. This season, a quantity of influenza vaccine similar to 1999--2000 will be available; however, vaccine distribution has been delayed (3,4). This delay may have limited the opportunity for vaccination of persons at high risk for complications from influenza, household contacts of high-risk persons, and health-care providers who care for high-risk persons; therefore, vaccination efforts for these groups should continue during December, January, and beyond, if necessary. Efforts also should be made to vaccinate persons aged 50--64 years. Un-vaccinated persons can benefit from influenza vaccination even after influenza activity has begun in their community.

As of December 4, approximately 51.2 million (68%) of the 75 million doses of influenza vaccine projected to be produced this year had been distributed. CDC has contracted with Aventis Pasteur to produce 9 million of the 75 million doses, and this vaccine will be available for distribution beginning in mid-December (5). Information on vaccine prices and ordering procedures is available on the World-Wide Web, The deadline for placing applications for orders is December 15, 2000. As of December 4, applications had been received for approximately 46% of this vaccine.

Four prescription antiviral medications are approved for treating uncomplicated influenza: Amantadine is approved to treat influenza A in persons aged >1 year, rimantadine for treating influenza A in adults, Zanamivir for treating influenza A and B in persons aged >7 years, and Oseltamivir for treating influenza A and B in persons aged >18 years. These four antiviral agents can reduce the duration of influenza symptoms by approximately 1 day if treatment is started within 48 hours of symptom onset, but the agents differ in routes of administration, contraindications, adverse effects, and cost. Three antiviral medications are approved for chemoprophylaxis of influenza but are not substitutes for influenza vaccination. Amantadine and rimantadine are approved for chemoprophylaxis of influenza A in persons aged >1 year. Oseltamivir recently was approved for chemoprophylaxis of influenza A and B in persons aged >13 years. Chemoprophylactic use of antiviral drugs can be helpful in controlling influenza outbreaks in specific situations (e.g., in long-term--care facilities). Long-term antiviral chemoprophylaxis also might be indicated for high-risk institutionalized persons or persons at high risk for complications from influenza if vaccine is unavailable, ineffective (e.g., in severely immunocompromised persons), or contraindicated. Widespread use of antiviral drugs as chemoprophylaxis for influenza is not recommended.

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 number 361100), or on the World-Wide Web,


  1. CDC. Influenza activity---United States, 1999--2000 season. MMWR 1999;48:1039--42.
  2. Simonsen L, Fukuda K, Schonberger LB, Cox NJ. The impact of influenza epidemics on hospitalizations. J Infect Dis 2000;181:831--7.
  3. CDC. Updated recommendations from the Advisory Committee on Immunization Practices in response to delays in supply of influenza vaccine for the 2000--01 season. MMWR 2000;49:888--92.
  4. CDC. Delayed supply of influenza vaccine and adjunct ACIP influenza vaccine recommendations for the 2000--01 influenza season. MMWR 2000;49:619--22.
  5. CDC. CDC contract for additional 9 million doses of influenza vaccine for the 2000--01 season. MMWR 2000;49:999.

* The four components of the influenza surveillance system have been described (1). Information reported as of November 30, 2000.

Temperature >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.

Before the 1999--2000 season, the case definition for P&I deaths was modified. CDC analysis estimated that the revised case definition resulted in an average increase in baseline P&I mortality estimates of 0.8% for 1999--2000. Thus, the 122 cities P&I mortality baseline and epidemic threshold for the 2000--01 season have been adjusted upward. 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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