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Update: Influenza Activity -- United States and Worldwide, 1995-96 Season, and Composition of the 1996-97 Influenza Vaccine

To monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses, CDC conducts surveillance in collaboration with the World Health Organization (WHO) and its international network of collaborating laboratories and with state and local health departments in the United States. This report summarizes surveillance for influenza in the United States and worldwide during the 1995-96 season and describes the composition of the 1996-97 influenza vaccine.

United States

Influenza activity began in November 1995 and peaked during late December 1995 and early January 1996. In many parts of the country, influenza activity declined steadily during January and February; of the 34 states that reported levels of influenza-like illness for the week ending April 13, a total of 16 states reported sporadic * levels of influenza-like illness, and 18 states reported no activity.

Of the 4132 influenza virus isolates reported to CDC from WHO collaborating laboratories in the United States from October 1, 1995, through March 30, 1996, a total of 3786 (92%) were influenza type A and 346 (8%) influenza type B. Of the 2416 type A isolates that were subtyped, 1427 (59%) were type A(H1N1), and 989 (41%) were type A(H3N2). Influenza type A(H3N2) predominated in the Mountain, New England, and Pacific regions, accounting for 70%, 56%, and 55% of subtyped influenza A isolates, respectively. Influenza type A(H1N1) predominated in the other six regions, accounting for 55%-82% of subtyped influenza A isolates. During February, although the total number of isolates decreased, the number and proportion of influenza type B isolates began to increase, and during March 1996, 50%-72% of all isolates reported were type B.

The proportion of all deaths reported by the vital statistics offices of 121 U.S. cities that were attributed to pneumonia and influenza (P&I) only slightly exceeded the epidemic threshold ** during 3 of the 8 weeks from October 29 through December 23, 1995. During the 6 weeks from December 24, 1995, through February 3, 1996, the proportion of P&I deaths remained above the epidemic threshold, peaking at 8.2% of all deaths during the week ending January 20. However, since February 10, percentages of P&I deaths have been below the epidemic threshold.


Influenza activity occurred at moderate to severe levels during October 1995-March 1996. Epidemics associated with influenza A(H3N2) and A(H1N1) viruses were reported in countries in Europe, Asia, and North America, while influenza B viruses circulated at low levels.

School outbreaks caused by influenza A(H3N2) viruses were reported in England beginning in September and October. During November and December, epidemic activity associated primarily with A(H3N2) viruses was reported by countries throughout Europe, including Belarus, Bulgaria, Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Latvia, Netherlands, Norway, Slovakia, Spain, Sweden, and the United Kingdom. During January, influenza A(H3N2) viruses were associated with outbreaks in Beijing and with high levels of influenza-like illness in six northern provinces of China. Isolation of influenza A(H3N2) viruses also was reported in North America (Canada and the United States), Europe (Belgium, Iceland, Ireland, Italy, Poland, Portugal, the Russian Federation, and Switzerland), Asia (Guam, Hong Kong, Japan, and Singapore), and Oceania (Australia and New Zealand). For the first time since the 1991-92 influenza season, A(H1N1) viruses were associated with epidemics in several regions of the world. Epidemic activity associated with influenza A(H1N1) viruses was reported and predominated in Belgium, Canada, Japan, southern France, Switzerland, and the United States. Influenza A(H1N1) viruses were isolated in association with sporadic activity in Europe (Finland, Germany, Italy, Latvia, Netherlands, Poland, Romania, Russian Federation, Spain, Sweden, and the United Kingdom) and Asia (China, Hong Kong, Israel, and Thailand).

In comparison to type A influenza viruses, type B viruses have been isolated later in the season and less frequently worldwide. Influenza B viruses were isolated primarily in association with sporadic activity in North America (Canada and the United States), Asia (China, Hong Kong, Israel, Japan, and Singapore), Europe (Belarus, Finland, France, Germany, Greece, Hungary, Netherlands, Poland, Romania, Russian Federation, Sweden, Switzerland, and the United Kingdom), and Oceania (Australia and New Zealand).

Composition of the 1996-97 Vaccine

The Food and Drug Administration Vaccines and Related Biological Products Advisory Committee (VRBPAC) has recommended that the 1996-97 trivalent influenza vaccine for the United States contain A/Wuhan/359/95-like(H3N2), A/Texas/36/91-like (H1N1), and B/Beijing/184/93-like viruses. This recommendation was based on the antigenic analysis of recently isolated influenza viruses and the antibody responses of persons vaccinated with the 1995-96 vaccine.

Although most of the influenza type A(H3N2) viruses that have been antigenically characterized are similar to the A/Johannesburg/33/95 strain, increasing numbers of recently isolated A(H3N2) strains from Asia, Europe, and North America are more similar to the antigenic variant A/Wuhan/359/95 (Table_1). Vaccines containing the A/Johannesburg/33/94(H3N2)-like virus induced a good antibody response to the vaccine strain but induced lower and less frequent antibody responses to recent type A(H3N2) strains such as A/Wuhan/359/95 (1). Therefore, VRBPAC recommended changing the influenza type A(H3N2) vaccine component to an A/Wuhan/359/95-like strain for the 1996-97 season. The strain that will be used by U.S. vaccine manufacturers because of its growth properties will be the antigenically equivalent A/Nanchang/933/95 virus.

Virtually all (98%) influenza A(H1N1) viruses that have been antigenically characterized are similar to the reference strains A/Taiwan/01/86 and A/Texas/36/91. Because vaccines containing the A/Texas/36/91 strain induced antibodies with similar frequency and titer to both the vaccine virus and to recent type A(H1N1) strains (1), VRBPAC recommended retaining an A/Texas/36/91-like strain in the 1996-97 vaccine.

Antigenically characterized influenza B viruses isolated recently in Asia, Europe, and the United States have been similar to the reference strains B/Beijing/184/93 and B/Harbin/07/94. Vaccines containing the B/Harbin/07/94 strain induced antibodies with similar frequency and titer to the vaccine virus and to recently isolated influenza B strains (1). Therefore, VRBPAC recommended retaining B/Harbin/07/94-like strain in the 1996-97 vaccine.

Reported by: Participating state and territorial health dept epidemiologists and state public health laboratory directors. M Zambon, PhD, Central Public Health Laboratory, A Hay, PhD, National Institute for Medical Research, London; G Schild, DSc, J Wood, PhD, National Institute for Biological Standards and Control, Hertfordshire, England. I Gust, MD, A Hampson, Commonwealth Serum Laboratories, Parkville, Australia. L Canas, Armstrong Laboratory, Brooks Air Force Base, Texas. Y Guo, Institute of Virology, National Center for Preventive Medicine, Beijing, People's Republic of China. World Health Organization National Influenza Centers, Div of Emerging and other Communicable Diseases Surveillance and Control, Geneva. Div of Virology, Center for Biologics Evaluation and Research, Food and Drug Administration. Influenza Br, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: During the 1995-96 season, the impact of influenza in many parts of the United States and in some other countries in the Northern Hemisphere was more severe than during the previous season (2). In the United States, influenza type A(H1N1) viruses predominated for the first time since the 1986-87 season; although this subtype has not been associated with excess mortality in recent decades, the incidence of infection with type A(H1N1) has been high, especially among school-aged children. Influenza type A(H3N2) was not the predominant strain but circulated throughout the season and was associated with outbreaks among all age groups. Continued circulation of influenza type A(H3N2) and type A(H1N1) is anticipated during the 1996-97 season. Influenza B activity increased late in the 1995-96 influenza season, suggesting that type B viruses may circulate more widely next winter.

Strains to be included in the influenza vaccine usually are selected during the preceding January through March because of scheduling requirements for production, quality control, packaging, and distribution of vaccine for administration before onset of the next influenza season. Recommendations of the Advisory Committee on Immunization Practices for the use of vaccine and antiviral agents for prevention and control of influenza will be published in an MMWR Recommendations and Reports on May 3, 1996.


  1. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1996-1997 flu season. Wkly Epidemiol Rec 1996; 71:57-61.

  2. CDC. Update: influenza activity -- United States and worldwide, 1994-95 season, and composition of the 1995-96 influenza vaccine. MMWR 1995;44: 292-5.

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 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 calculated using a periodic regression model applied to observed percentages since 1983. The baseline was calculated using a robust regression procedure.
Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 1. Hemagglutination-inhibition titers of influenza A(H3N2) viruses with serum
specimens from infected ferrets *
                                            Ferret antiserum
Viral antigen            A/Johannesburg/33/94   A/Alaska/10/95   A/Wuhan/359/95
Reference antigen
  A/Johannesburg/33/94           1280                 160               80
  A/Alaska/10/95                  320                1280              640
  A/Wuhan/359/95                   80                 160              640

Recent isolates
  A/Missouri/017/96              1280                 320               40
  A/England/409/95               1280                 320               80
  A/Nanchang/933/95               160                 320             1280
  A/Japan/368/96                   80                 320              640
  A/New York/09/96                160                 320             1280
  A/Washington/416/96              80                 320              640
* A fourfold difference in hemagglutination-inhibition titers with two viruses is usually indicative
  of antigenic variation between viruses.

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