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Update: Influenza Activity -- United States, 1996-97 Season

In collaboration with the World Health Organization (WHO), its collaborating laboratories, and state and local health departments, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. This report summarizes influenza activity in the United States from September 29 through December 7, 1996, and indicates that influenza activity has increased since mid-November.

From September 29 through December 7, a total of 602 (7%) of 8927 respiratory specimens tested by WHO collaborating laboratories in the United States were positive for influenza virus. Of the 602 positive cultures, 458 (76%) were reported during November 17-December 7. Of the 602 influenza isolates, 595 (99%) were influenza type A, and seven (1%) were type B. Of the 158 influenza type A isolates subtyped, all were A(H3N2). CDC further characterized 13 influenza A(H3N2) isolates, and all were antigenically similar to the H3N2 component of the 1996-97 influenza vaccine.

Regional influenza activity * was first reported by state or territorial epidemiologists during the week ending October 19 in Maryland and continued to be reported from one to two states each week through the week ending November 16; regional activity increased to seven, 10, and 14 states for the weeks ending November 23, November 30, and December 7, respectively. Widespread activity was first reported in Colorado and Pennsylvania during the week ending November 23, and in Connecticut during the week ending December 7. For the first time this season, the percentage of patients with influenza-like illness who visited sentinel physicians (5% for the week ending December 7) exceeded baseline levels (0-3%).

During September 29-December 7, the percentage of deaths attributed to pneumonia and influenza (P&I) as reported by the vital statistics offices of 121 cities has not exceeded the epidemic threshold **. Peaks in physician-reported influenza morbidity typically precede peaks in P&I mortality by 2-5 weeks (1). As of December 7, a total of 23 outbreaks of influenza were reported to CDC; five (one in Indiana and two each in New York and Wisconsin) are described here. Indiana

During November 24-December 1, a total of 431 (65%) of 660 persons aged 15-24 years attending a vocational training camp in Indiana had onset of acute febrile respiratory manifestations (fever greater than or equal to 100 F { greater than or equal to 38 C} and cough); 20 (10%) of 200 staff had onset of similar illness. On November 27, three of four nasopharyngeal swab specimens obtained from ill persons were positive for influenza type A by rapid antigen-detection test. At the state public health laboratory, three of these specimens were further subtyped as influenza A(H3N2). Before the outbreak, 56 (9%) camp attendees and approximately 90% of the staff had been vaccinated with the 1996-97 influenza vaccine. During the Thanksgiving break, all persons who were asymptomatic left the camp, and symptomatic persons remained at the camp. After all attendees returned to the camp on December 1, only three additional cases were identified. New York

During October 1-19, a total of 38 (50%) of 76 residents of two of the 17 units in a 571-bed long-term-care facility in Rochester, New York, had onset of acute febrile respiratory manifestations. Of the 442 employees of the facility who responded to a questionnaire, 46 (10%) had onset of similar illness. Of the nasopharyngeal swab specimens obtained from 15 ill residents, six (40%) were positive for influenza type A at the state public health laboratory; four of these isolates were sent to CDC and further characterized as influenza A/Wuhan/359/95-like(H3N2). None of the residents or staff had received influenza vaccine before the outbreak. A total of 529 (93%) residents and 581 (75%) of 780 employees were vaccinated during the outbreak. Residents of the two affected units were isolated from residents of other units during October 4-18 and October 10-25, respectively. One resident was hospitalized with pneumonia. New York City

During November 6-16, a total of 25 (5%) of 499 residents of a nursing home had onset of acute respiratory manifestations. All ill residents were living on seven floors of the 13-floor facility. Fifteen (2%) of 650 employees also had onset of similar illness. Of nasopharyngeal swab specimens obtained from eight ill residents, five were positive for influenza type A by a rapid antigen detection test. One of these specimens was cultured and confirmed as influenza type A. During October 1996, a total of 427 (91%) of 470 residents for whom information was available had received influenza vaccine. During November 7-13, residents of the two floors affected first were isolated from residents of other floors, and the staff assigned to these floors did not circulate to other floors. In addition, on November 8, amantadine was administered to all 80 residents on those two floors. On November 12, amantadine was administered to all other asymptomatic residents. Nine patients were hospitalized; two developed pneumonia, and seven had exacerbation of underlying medical conditions. Wisconsin

On September 19, an outbreak of acute febrile respiratory illness began among students at a university in Milwaukee. Of the nasopharyngeal swab specimens obtained from 295 ill students, 47 were positive for influenza type A. Amantadine was administered to severely ill students. As of December 6, the outbreak was ongoing.

On November 25, an outbreak of acute febrile respiratory illness began among students at another university in Milwaukee. Seven (47%) of the 15 nasopharyngeal specimens cultured were positive for influenza type A. As of December 6, the outbreak was still in progress.

Ten isolates from the outbreaks in Milwaukee were identified as influenza A(H3N2) virus; one of these was antigenically characterized by CDC as influenza A/Wuhan/359/ 95-like(H3N2).

Reported by: G Steele, PhD, Epidemiology Resource Center and State Epidemiologist, D Bixler, MD, M Yonker, MA, Virology Laboratory, Indiana State Dept of Health; M Schultz, G Atterbury, Job Corps; S Allen, MD, Indiana Univ Medical Center, Indianapolis. J Davis, MD, State Epidemiologist for Communicable Diseases, T Haupt, Div of Health, Wisconsin Dept of Health and Social Svcs; G Sedmak, PhD, City of Milwaukee Health Dept. L Cheney, Monroe Community Hospital, Rochester; L Villarente, DeWitt Nursing Home, New York City; D Ackman, MD, Bur of Communicable Disease Control, P Bernstein, MD, R Kirshenbaum, DO, S Kondracki, G Balzano, Regional Epidemiologist, S Switzer, G Brady, P Smith, MD, State Epidemiologist, New York State Dept of Health. Participating state and territorial epidemiologists and state public health laboratory directors. World Health Organization collaborating laboratories. Influenza Br and World Health Organization Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: During the 1996-97 influenza season, 99% of influenza viruses identified have been type A, and all subtyped isolates have been type A(H3N2). Because influenza A(H3N2) has been associated with increased morbidity and mortality among the elderly (2-4), nursing homes should routinely offer all residents influenza vaccine and should develop contingency plans for rapid administration of amantadine or rimantadine during suspected or confirmed influenza type A outbreaks. The recommended period for influenza vaccination is from the beginning of October through mid-November (5). However, influenza vaccine should continue to be offered to unvaccinated persons at high risk for complications even after influenza activity is documented in a community. During influenza type A outbreaks, amantadine or rimantadine can be administered for the 2-week period following vaccination to provide protection until vaccine-induced antibody has developed (5). Amantadine and rimantadine also are effective for treatment of influenza type A if initiated within 48 hours of illness onset.

Throughout the influenza season, surveillance data collected by CDC is updated weekly and is available through the CDC voice information system, telephone (404) 332-4551, or the fax information system, telephone (404) 332-4565, by requesting document number 361100. Information about local influenza activity is available from many county and state health departments.


  1. Buffington J, Chapman LE, Schmeltz LM, Kendal AP. Do family physicians make good sentinels for influenza? Arch Fam Med 1993;2:859-64.

  2. Lui KL, Kendal AP. Impact of influenza epidemics on mortality in the United States from October 1972 to May 1985. Am J Public Health 1987;77:712-6.

  3. Noble GR. Epidemiological and clinical aspects of influenza. In: Beare AS, ed. Basic and applied research. Boca Raton, Florida: CRC Press, 1982:11-50.

  4. CDC. Update: influenza activity -- United States, 1995-96 season. MMWR 1996;45:134-6.

  5. ACIP. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1996;45(no. RR-5).

* Levels of activity are 1) no activity; 2) sporadic -- sporadically occurring influenza-like illness (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 less than 50% of the state's total population; and 4) widespread -- outbreaks of ILI or culture-confirmed influenza in counties with a combined population of greater than or equal to 50% of the state's total 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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