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Update: Influenza Activity -- United States, 1995-96 Season

Influenza activity in the United States increased steadily from late October through mid-December 1995. This report summarizes influenza surveillance data from October 1 through December 16, 1995.

During October 1-December 16, influenza viruses were isolated in 45 states and the District of Columbia. Of the 296 influenza virus isolates reported by World Health Organization (WHO) collaborating laboratories in the United States, 293 (99.9%) were type A and three (0.1%) were type B. Of the type A isolates, 140 (48%) were not subtyped. Of the 153 subtyped viruses reported, 91 (59%) were type A(H1N1), and 62 (41%) were type A(H3N2). Forty-two (82%) of 51 of the type A(H3N2) virus isolates and 46 (57%) of 81 of the type A(H1N1) isolates submitted to CDC for antigenic characterization were tested. More than 90% of these viruses were antigenically similar to the type A strains included in the 1995-96 influenza vaccine.

The number of states reporting regional or widespread * influenza-like illness (ILI) increased each week from the week ending November 18 (four states) through the week ending December 16 (29 states). Most reported outbreaks of ILI or culture-confirmed influenza occurred among school-aged children. CDC's sentinel physician surveillance system also indicated increasing influenza activity during the same period. The proportion of patients with ILI examined by sentinel physicians increased from mid-November through mid-December. During the first 2 weeks of December, an average of 6% of patient visits were for ILI.

Of total deaths reported through CDC's 121-city mortality surveillance system, the proportion of deaths associated with pneumonia and influenza exceeded the epidemic threshold ** during 3 of the 6 weeks from November 5 through December 16, 1995.

Reported by: Participating state and territorial epidemiologists and state public health laboratory directors. World Health Organization collaborating laboratories. Sentinel Physicians Influenza Surveillance System of the American Academy of Family Physicians, and Influenza Br, and WHO Collaborating Center for Influenza Surveillance, Epidemiology, and Control of Influenza, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The findings in this report indicate a steady increase in influenza activity from October 29 through December 16, with influenza type A(H1N1) viruses predominating. Before this influenza season, influenza type A(H1N1) viruses had not circulated widely in the United States since the 1988-89 season, when they represented almost 50% of influenza virus isolates reported by WHO collaborating laboratories in the United States (1).

Human influenza type A(H1N1) viruses circulated worldwide from approximately 1920 to 1957, when they disappeared and were replaced by a new type A strain, type A(H2N2). In 1968, type A(H2N2) viruses disappeared and were replaced by the type A(H3N2) strain. In 1977, type A(H1N1) viruses reemerged and, since then, have cocirculated with type A(H3N2) viruses. Because the type A(H1N1) strain that appeared in 1977 was nearly identical to a strain that circulated in 1950 (2), most persons born before 1950 had preexisting immunity to this strain. Since 1977, epidemics caused by type A(H1N1) viruses have affected primarily persons born after the mid-1950s. During type A(H1N1) epidemics since 1977, elderly persons usually have not been severely affected, and, because approximately 90% of influenza-related deaths occur among persons aged greater than or equal to 65 years (3), such epidemics have not been associated with high mortality. However, as type A(H1N1) viruses continue to undergo antigenic variation (which occurs among all influenza viruses), the effect of these strains among elderly persons may change.

Since their emergence in 1968, influenza type A(H3N2) viruses have affected persons of all ages. Both type A strains have been associated with outbreaks in schools and colleges and increases in absenteeism in the workplace among younger adults, but since the reemergence of type A(H1N1) in 1977, type A(H3N2) strains have been substantially more likely than type A(H1N1) to cause serious illness among the elderly and outbreaks in nursing homes associated with high rates of medical complications. Rapid antigen-detection tests can detect influenza type A virus but cannot distinguish between the subtypes. Although laboratory tests that distinguish between the two influenza A subtypes are not widely available, many state health department laboratories are able to subtype these viruses.

Although antigens included in the 1995-96 influenza vaccine closely match influenza type A viruses characterized through December 16, antiviral agents should be considered as an adjunct to vaccination (4). The antiviral agents amantadine and rimantadine are effective against virtually all naturally occurring influenza type A strains and can be used to prevent or treat influenza type A infections. When influenza vaccine is given after influenza activity has begun in a community, these drugs may be administered to provide protection during the 1-2 weeks required for the development of vaccine-induced antibody. These agents also may be administered for the duration of influenza A activity to prevent infection in persons expected to have an inadequate antibody response (e.g., persons with severe immunosuppression or for whom influenza vaccine is contraindicated). In addition, amantadine and rimantadine should be used in nursing homes and other health-care facilities to prevent and control influenza A outbreaks. Because of differences in the pharmacokinetic properties of the two drugs, the dosage recommendations and the potential for adverse reactions vary with such factors as patient's age, presence of certain underlying health conditions, and potential for adverse drug interactions (4).

Influenza surveillance data are updated throughout the influenza season, and summaries are available by computer to subscribers of the Public Health Network and to health-care providers and the public through the CDC Voice Information System, telephone (404) 332-4555; or facsimile, (404) 332-4565.


  1. CDC. Influenza -- United States, 1988-89. MMWR 1993;42(no. SS-1).

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

  3. Lui KJ, 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.

  4. ACIP. Prevention and control of influenza: recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR 1995;44(no. RR-3).

Levels of activity are 1) sporadic -- sporadically occurring influenza-like illness (ILI) or culture-confirmed influenza with no outbreaks detected; 2) 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 3) widespread -- outbreaks of ILI or culture-confirmed influenza in counties having 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 calculated using a periodic regression model applied to observed percentages since 1983. The baseline was calculated using a robust regression procedure.

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