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Influenza Activity --- United States, 2001--02 Season

In collaboration with the World Health Organization (WHO) and its collaborating laboratories, National Respiratory and Enteric Virus Surveillance System (NREVSS) collaborating laboratories, state and local health departments, and a network of sentinel physicians, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in circulating strains of influenza viruses. This report summarizes influenza activity in the United States* (1) during September 30--November 24, 2001, when the viruses isolated most frequently were influenza A (H3N2). These viruses were well matched antigenically by the 2001--02 influenza A (H3N2) vaccine. Vaccine supplies are plentiful and influenza vaccine should continue to be offered during December and later.

As of November 24, WHO and NREVSS collaborating laboratories in the United States tested 8,140 specimens for influenza viruses; 73 (0.9%) were positive. The percentage of positive influenza isolates identified each week is an indicator of the level of influenza activity, and for the weeks ending October 6 through November 24, the percentage of respiratory specimens testing positive for influenza viruses ranged from 0.4% to 1.7%. These percentages are low compared with the 24%--33% testing positive at the peak of the 1998--99, 1999--2000, and 2000--01 seasons. Of the 73 influenza isolates reported since September 30, 70 (96%) were influenza A viruses and three (4%) were influenza B viruses. Of the 70 influenza A viruses identified, 45 (64%) have been subtyped; 44 were influenza A (H3N2) viruses and one was an influenza A (H1N1) virus. Influenza A (H3N2) isolates were identified in Alaska, Arizona, Colorado, Florida, New York, North Carolina, North Dakota, Texas, Utah, and Wisconsin. The influenza A (H1N1) isolate was identified in Washington, and unsubtyped influenza A isolates were identified in Alabama, Alaska, Hawaii, Louisiana, Minnesota, New York, Washington, and Wisconsin. Influenza B isolates were identified in Louisiana, Michigan, and Texas. Thirty-nine (52%) of the 73 influenza viruses isolated were identified in Alaska.

CDC antigenically characterized 10 influenza isolates collected in September and 13 collected in October. They consisted of 20 influenza A (H3N2) viruses, two influenza A (H1N1) viruses, and one influenza B virus. The antigenically characterized influenza A (H3N2), influenza A (H1N1), and influenza B isolates were similar to the vaccine strains A/Panama/2007/99 (H3N2), A/New Caledonia/20/99 (H1N1), and B/Sichuan/379/99, respectively.

During September 30--November 24, the weekly percentage of patient visits for influenza-like illness (ILI) to approximately 650 U.S. sentinel physicians ranged from 1.0% to 1.4%. For the week ending November 24, the percentage of patient visits for ILI was 1.4%, which is less than the national baseline of 1.9%§. During the same week, influenza activity, as reported by state epidemiologists, was regional in Alaska and sporadic in 25 states (Alabama, Arizona, California, Colorado, Connecticut, Georgia, Indiana, Iowa, Kansas, Kentucky, Maine, Michigan, Missouri, Nevada, New Mexico, New York, North Carolina, Ohio, Tennessee, Texas, Utah, Vermont, West Virginia, Wisconsin, and Wyoming), New York City, and District of Columbia; 23 states reported no influenza activity, and one state did not report.

During the week ending November 24, the 122 Cities Mortality Reporting System attributed 6.1% of recorded deaths to pneumonia and influenza (P&I). This percentage was below the epidemic threshold** of 7.4% for that week. The percentage of P&I deaths has been below the epidemic threshold for each week since September 30.

In November, two virologically confirmed institutional outbreaks caused by influenza A viruses were reported to CDC. On November 14, an elementary school in Fort Collins, Colorado, reported elevated and increasing absenteeism among its students. Of 675 students, 53 (8%) were absent on November 14, 96 (14%) were absent on November 15, and 110 (16%) were absent on November 16. Baseline absenteeism on November 12--13 was 18--20 students. Two of the three specimens submitted to the state laboratory for viral culture were positive for influenza A (H3N2). The school remained open and a letter was sent to parents describing influenza symptoms and requesting that sick children be kept at home. Use of influenza antiviral agents was left to the discretion of the child's health-care provider and family. Nursing homes in the Fort Collins area were advised of influenza activity in the community and a broadcast facsimile outlining antiviral treatments available for influenza was sent to all primary-care providers.

On November 17, an influenza A outbreak was reported in a long-term--care facility with 160 residents located in the Hudson Valley region of New York; 14 residents and eight staff members had an influenza-like illness and four of six ill residents tested positive for influenza A by rapid antigen testing. On November 18, all residents began to receive antiviral medication and since then, no new cases of influenza-like illness in this facility have been reported. The facility received its order of influenza vaccine a week and a half before the outbreak and vaccinated residents on November 12--16.

Reported by: S Berns, Poudre School District; N Underwood, S Murray, A LeBailly, MD, Larimer County Dept of Health and Environment, Fort Collins; A Scott, K Gershman, MD, L Swanson, P Young, Colorado Dept of Public Health and Environment. C Waters, P Smith, MD, New York Dept of Health. 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 Surveillance, Epidemiology, and Control of Influenza, Influenza Br and Respiratory and Enteric Virus Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note:

The four influenza surveillance system components indicated low levels of influenza activity in the United States during September 30--November 24. The number of influenza viruses isolated this season is relatively low and it is too early to determine which strain(s) will predominate. However, two influenza A outbreaks were detected in November and influenza activity is expected to increase during the next few weeks to months. The viruses isolated most frequently have been influenza A (H3N2) viruses. The 2001--02 influenza vaccine strains are well matched to the influenza isolates that have been characterized antigenically this season.

The best prevention against influenza is vaccination. Vaccine supplies are plentiful and are available for immediate shipment from the three U.S. licensed manufacturers. Manufacturers estimate that approximately 87 million doses of influenza vaccine will be produced this year compared with 76.8 million doses available during the 1999--2000 season and 70.4 million doses available during the 2000--01 season. By the end of November, approximately 74.2 million (85%) of the projected 87 million doses of vaccine will have been distributed. An additional 12.8 million doses are expected to be available in December.

Health-care providers should continue to offer influenza vaccine during December and later because persons can benefit from vaccination after influenza activity has been detected in their community (2). The most important persons to be vaccinated are those in groups at increased risk for complications from influenza (i.e., persons aged >65 years and persons aged 6 months--64 years with certain underlying medical conditions [3]), and health-care providers. In addition, household contacts of high-risk persons, healthy persons aged 50--64 years, and anyone who wants to reduce the likelihood of becoming ill with influenza should be vaccinated.

CDC collects and reports U.S. influenza surveillance data during October--May. This information is updated weekly and is available through CDC voice information, 888-232-3228, fax information, 888-232-3299 (request document number 361100) or at


  1. CDC. Influenza activity---United States, 1999--2000 season. MMWR 1999;48:1039--42.
  2. CDC. Delayed influenza vaccine availability for the 2001--02 season and supplemental recommendations of the Advisory Committee on Immunization Practices. MMWR 2001;50:582--5.
  3. CDC. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 2001;50(no. RR-4).

*As of November 29, 2001. .

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

§ 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, to calculate region-specific baselines is not possible and to apply the national baseline to regional level data is not appropriate.

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.

** 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 since 1983. The epidemic threshold is 1.654 standard deviations above the seasonal baseline. Before the 1999--2000 season, a new case definition for a P&I death was introduced. During the summer of 2000, the baseline and epidemic thresholds were adjusted manually to account for these changes in case definition. For the 2001--02 season, sufficient data have been collected using the new case definition to allow projection of the baseline using the regression procedure employed before the 2000--01 season.

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