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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 activity from October 4 through November 7, 1998; during this period, overall influenza activity in the United States was low. However, outbreaks of influenza A occurred in New York during October-November.

As of November 7, the 110 WHO and National Respiratory Enteric Virus Surveillance System collaborating laboratories in the United States have tested 3394 clinical specimens (by culture or direct antigen-detection techniques) for respiratory viruses. Of these, 16 (0.5%) were influenza A, and two (0.1%) were influenza B. The six influenza A isolates collected since October 4 and antigenically characterized by CDC were related to A/Sydney/05/97(H3N2), the H3N2 component in the 1998-99 influenza vaccine, and the two influenza B isolates that were characterized by CDC were related to B/Beijing/184/93, the B component in the 1998-99 influenza vaccine.

Regional * influenza activity was reported by New York (weeks ending October 17 and November 7) and by Maryland (weeks ending October 17 and October 31). Outbreaks of influenza A occurred in three nursing homes in Bronx and Nassau counties, New York. Through the week ending November 7, no other state and territorial epidemiologists reported regional influenza activity. The percentage of patient visits to sentinel physicians for influenza-like illness remained within baseline levels (0-3%) since the week ending October 4, and the percentage of deaths attributed to pneumonia and influenza reported by the vital statistics offices of 122 cities has not exceeded the epidemic threshold ** during consecutive weeks through the week ending November 7.

Reported by: S Kondracki, H Leib, 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. National Respiratory Enteric Virus Surveillance System collaborating laboratories. Sentinel Physicians Influenza Surveillance System. WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The influenza A(H3N2) and influenza B viruses isolated in the United States during this season are similar to those isolated worldwide during the previous 6 months and to the A(H3N2) and B components in the 1998-99 influenza vaccine. During this season, no influenza A(H1N1) viruses have been identified in the United States. Few H1N1 isolates have been identified worldwide during the previous 6 months.

Although the optimal time for influenza vaccination is October through mid-November, health-care providers should continue to offer influenza vaccine to unvaccinated high-risk persons after mid-November, even if influenza activity has been detected in the community. The Advisory Committee on Immunization Practices recommends annual vaccination against influenza for persons aged greater than or equal to 65 years; residents of nursing homes or chronic-care facilities; persons with chronic cardiovascular or pulmonary disorders, including children with asthma; persons requiring medical follow-up or hospitalization during the previous year because of diabetes or other chronic metabolic diseases, renal dysfunction, hemoglobinopathies, or immunosuppression; children and teenagers (aged 6 months-18 years) receiving long-term aspirin therapy (who may therefore be at risk for developing Reye syndrome after influenza); and women who will be in their second or third trimester of pregnancy during the influenza season. Vaccination also is recommended for health-care workers and others, including household members, in frequent contact with persons at high risk for influenza-related complications. Influenza vaccine also can be administered to other persons who want to reduce their likelihood of acquiring influenza and for whom vaccination is not contraindicated (1).

Antiviral agents can provide a useful adjunct to influenza vaccination (1). Amantadine and rimantadine are available for the prophylaxis or treatment of influenza A infection, but they are not effective against influenza type B viruses. In settings that house persons at high risk for influenza-related complications (e.g., nursing homes), contingency plans for rapid diagnostic testing for influenza type A viruses can help detect outbreaks early and guide use of antiviral drugs for prophylaxis and treatment (2).

Throughout the influenza season, surveillance data collected by CDC are updated weekly and are available through CDC's voice information system, telephone (888) 232-3228, or fax information system, telephone (888) 232-3299, by requesting document no. 361100, or through CDC's World-Wide Web site ncidod/diseases/flu/weekly.htm.


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

  2. 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 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 pneumonia and influenza since 1983.

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