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Update: Influenza Activity -- United States, 1998-99 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 surveillance in the United States from October 4, 1998, to January 9, 1999, which indicates that overall influenza activity was low.

As of January 9, the 110 WHO and National Respiratory Enteric Virus Surveillance System collaborating laboratories in the United States had tested 20,972 specimens (by culture or direct antigen-detection techniques) for respiratory viruses. Of these, 401 (2%) were positive for influenza viruses; 293 (73%) were influenza A, and 108 (27%) were type B. Of the 95 (32%) influenza A isolates that have been subtyped, 93 (98%) were influenza A(H3N2) and two (2%) were influenza A(H1N1). Since October 4, all of the influenza A(H3N2) viruses antigenically characterized by CDC were similar to A/Sydney/5/97, the H3N2 component of the 1998-99 influenza vaccine. One influenza A(H1N1) isolate was antigenically characterized as an A/Bayern/7/95-like virus that is antigenically distinct from A/Beijing/262/95, the H1N1 vaccine strain. However, the 1998-99 A(H1N1) vaccine strain produces high titers of antibodies that cross react with A/Bayern/7/95 (1). All 15 of the influenza B viruses antigenically characterized by CDC are similar to B/Beijing/184/93, the recommended type B vaccine strain.

Since October 4, 1998, 41 states have reported laboratory-confirmed influenza. Influenza A(H3N2) viruses were reported from 24 states, influenza A(H1N1) viruses from two states, influenza B viruses from 26 states, and influenza A (not subtyped) viruses from 32 states. For the week ending January 9, 1999, New York City reported widespread * influenza activity, 10 states reported regional activity, and 35 states reported sporadic activity. The overall percentage of patient visits to sentinel physicians for influenza-like illness remained within baseline levels (0-3%) during the entire period. The percentage of deaths attributed to pneumonia and influenza reported by the 122 Cities Pneumonia and Influenza Mortality Surveillance System ranged from 6% to 7% and intermittently exceeded the epidemic threshold ** for a combined total of 5 of 14 weeks, but has not remained above the epidemic threshold for greater than 2 consecutive weeks.

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. World Health Organization collaborating laboratories, National Respiratory Enteric Virus Surveillance System collaborating laboratories. Sentinel Physicians Influenza Surveillance System. WHO Collaborating Center for Reference and Research on Influenza, Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The findings in this report indicate that, despite institutional outbreaks in several states, this influenza season has been relatively mild. However, influenza activity has increased since mid-December and may increase during subsequent weeks. Although the optimal time for influenza vaccination is October through mid-November, influenza vaccine should still be offered to unvaccinated high-risk persons, health-care providers, caregivers, and household contacts of high-risk persons even after influenza activity has been detected in the community.

All influenza A strains and most influenza B strains isolated in the United States that have been characterized by CDC are well matched by the 1998-99 influenza vaccine. A/Sydney/5/97-like (H3N2) viruses were the predominant influenza viruses isolated in the United States during the 1997-98 season and were isolated throughout 1998 (2-4). Even when the match between circulating strains and vaccine strains is good, outbreaks of influenza can still occur in vaccinated persons. Therefore, use of the antiviral agents amantadine and rimantadine in addition to influenza vaccination may help prevent and control influenza A but not influenza B, especially among persons at high risk for influenza-related complications and in institutions such as nursing homes (5,6). These drugs are 70%-90% effective in preventing influenza A infections and reduce the severity and duration of symptoms when administered within 48 hours of illness onset.

Commercially available point-of-care rapid diagnostic tests for influenza include one test that detects only influenza A virus and two tests that detect both influenza A and B viruses but do not distinguish between the infections. Rapid diagnostic tests for influenza in institutional outbreaks are most useful when used in conjunction with viral cultures. Rapid identification of influenza virus infection is important because prevention measures, such as cohorting and isolating infected and symptomatic persons, can be implemented more quickly.

Influenza surveillance data are updated weekly throughout the season. Summary reports are available through CDC's voice information system, (888) 232-3228, or fax information system, (888) 232-3299, by requesting document number 361100 and entering the telephone number to which the document should be transmitted, or through CDC's National Center for Infectious Diseases, Division of Viral and Rickettsial Diseases, Influenza Branch World-Wide Web site diseases/flu/weekly.htm.


  1. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1998-1999 season. Wkly Epidemiol Rec 1998;73:9:56-63.

  2. CDC. Update: influenza activity -- United States and worldwide, 1997-98 season, and composition of the 1998-99 influenza vaccine. MMWR 1998;47:14:280-4.

  3. CDC. Update: outbreak of influenza A infection -- Alaska and the Yukon Territory, June-July 1998. MMWR 1998;47:30:638.

  4. CDC. Influenza A -- Florida and Tennessee, July-August 1998, and virologic surveillance of influenza, May-August 1998. MMWR 1998;47:36:756-9.

  5. CDC. Prevention and Control of Influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1998;47(no. RR-6).

  6. Gomolin IH, Leib HB, Arden NH, Sherman FT. Control of influenza outbreaks in the nursing home: guidelines for diagnosis and management. J Am Geriatr Soc 1995;43:71-4.

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 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 percentage of deaths from pneumonia and influenza since 1983.

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