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Update: Influenza Activity -- United States and Worldwide, 1994-95 Season, and Composition of the 1995-96 Influenza Vaccine

In collaboration with the World Health Organization (WHO) and the international network of collaborating laboratories and with state and local health departments in the United States, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza viruses. This report summarizes surveillance for influenza in the United States and worldwide during the 1994-95 season and describes the composition of the 1995-96 influenza vaccine. United States

Influenza activity began in the Northeast in late November 1994 and from late January to early February spread to other regions of the country. Activity peaked during March and continues to decline.

From November 27, 1994, through January 14, 1995, regional or widespread influenza activity * was reported only from northeastern states. Regional activity was first reported outside this area for the week ending January 21, and by February 11 regional or widespread activity had been reported from every region in the country. Based on reports from state and territorial epidemiologists, peak activity occurred the week ending March 11, 1995, when 26 states reported either regional or widespread activity. The number of states reporting regional or widespread activity has declined every week since March 12. For the week ending April 8, four states reported regional activity, and none reported widespread activity.

Of total deaths reported through CDC's 121-city mortality surveillance system, the proportion attributed to pneumonia and influenza exceeded the epidemic threshold ** for 11 of the 27 weeks from October 2, 1994, through April 8, 1995. Pneumonia and influenza deaths exceeded the epidemic threshold for 2 consecutive weeks twice during this interval.

Of the 3423 influenza virus isolates reported to CDC from WHO collaborating laboratories in the United States through April 8, a total of 2654 (78%) were type A and 769 (22%) were type B. Of the 1337 type A viruses that have been subtyped, 1318 (99%) were type A(H3N2) and 19 (1%) were type A(H1N1). Worldwide

Influenza activity has occurred at low to moderate levels in most parts of the world. Although a few countries reported epidemic activity, sporadic activity or localized outbreaks were reported more frequently. Influenza activity was usually associated with cocirculation of influenza A(H3N2) and influenza B viruses. Influenza A(H1N1) activity was reported only in association with sporadic cases. Influenza A(H3N2) viruses were first detected during October in Europe and North America. Outbreaks associated with influenza A(H3N2) were subsequently reported in the People's Republic of China, Finland, Hungary, Italy, Spain, the United Kingdom, and the United States. Although influenza A and influenza B cocirculated, influenza A(H3N2) viruses predominated in Canada, Finland, France, Italy, Spain, and the United States.

Influenza type B viruses were first detected this season in Europe in association with a secondary school outbreak in Portugal during October. Outbreaks caused by influenza B were reported subsequently in China, Iran, Italy, and the United States. Epidemic activity associated with influenza B was reported in Italy and Russia. In Germany, the Netherlands, Portugal, Russia, and the United Kingdom, influenza B viruses were isolated more frequently than influenza A(H3N2) viruses.

Influenza A(H1N1) viruses have been reported in association with sporadic activity from Canada, China, Hong Kong, the Netherlands, Norway, Poland, Singapore, Switzerland, Thailand, the United Kingdom, and the United States during the 1994-95 season. Composition of the 1995-96 Vaccine

The Food and Drug Administration Vaccines and Related Biologicals Advisory Committee (VRBAC) has recommended that the 1995-96 trivalent influenza vaccine for the United States contain A/Johannesburg/33/94-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 1994-95 vaccine.

Although many of the influenza type A(H3N2) viruses that have been antigenically characterized are similar to the A/Shangdong/09/93 strain, some recently isolated A(H3N2) strains from Asia, Europe, and North America are more similar to the antigenic variant A/Johannesburg/33/94 Table_1. Vaccines containing the A/Shangdong/09/93(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/Johannesburg/33/94 (1). Therefore, VRBAC recommended changing the influenza type A(H3N2) vaccine component to an A/Johannesburg/33/94-like strain for the 1995-96 season.

Many recent influenza B viruses isolated from Asia, Europe, and North America are antigenically distinguishable from the B/Panama/45/90 strain included in the 1994-95 vaccine. These recent viruses are similar to the B/Beijing/184/93, B/Shanghai/04/94, and B/Harbin/07/94 strains. These strains, which are themselves antigenically indistinguishable, have been used as reference strains for antigenic analysis Table_2. Although vaccines containing B/Panama/45/90 virus induced antibodies at a similar frequency and titer as the vaccine virus for some recent influenza B strains, in some studies the antibody response in adults and the elderly was reduced to the B/Beijing/ 184/93-like strain, B/Shanghai/04/94. VRBAC recommended changing the influenza B component to a B/Beijing/184/93-like virus for the 1995-96 season. The actual strain used by U.S. vaccine manufacturers will be B/Harbin/07/94 because of its growth properties.

Since the 1992-93 influenza season, isolation of influenza type A(H1N1) virus has been sporadic worldwide (2). Nine recent viruses from China and the United States have been characterized as being related to the reference strains A/Taiwan/01/86 and A/Texas/36/91. Vaccines containing the A/Texas/36/91 strain induced antibodies with similar frequency and titer to the vaccine virus and to type A(H1N1) strains isolated in 1993 and 1994. Therefore, VRBAC recommended retaining an A/Texas/36/91-like strain in the 1995-96 vaccine. Reported by: Participating state and territorial health dept epidemiologists and state public health laboratory directors. M Chakraverty, PhD, Central Public Health Laboratory, A Hay, PhD, National Institute for Medical Research, London; G Schild, PhD, J Wood, PhD, National Institute for Biological Standards and Control, Hertfordshire, England. I Gust, MD, A Hampson, Commonwealth Serum Laboratories, Parkville, Australia. J Weber, Laboratory Center for Disease Control, Ottawa, Ontario. J Kim, PhD, K Park, PhD, National Institute of Health, Seoul, Korea. E Claas, PhD, Eramus University, Rotterdam, The Netherlands. World Health Organization National Influenza Centers, Program on Bacterial, Viral Diseases, and Immunology, 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 1994-95 season, the impact of influenza in most parts of the United States and in most other countries in the Northern Hemisphere was less severe than during the previous season, when A/Beijing/32/92-like (H3N2) viruses predominated. Although approximately 75% of influenza viruses circulating in the United States during the 1994-95 season have been type A(H3N2), compared with the 1993-94 season, influenza spread more slowly and was associated with less severe illness. The results of mortality surveillance based on the 121-city system suggest relatively low influenza-associated mortality in the United States this season and are consistent with other influenza surveillance findings.

Strains to be included in next season's influenza vaccine are selected usually 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 have been published in the MMWR Recommendations and Reports (3).


  1. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1995-96 season. Wkly Epidemiol Rec 1995;70:53-6.

  2. CDC. Update: influenza activity -- United States and worldwide, 1993-94 season, and composition of the 1994-95 influenza vaccine. MMWR 1994;43:179-83.

  3. CDC. 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 having 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. 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.

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/Shangdong/09/93   A/Guangdong/25/93   A/Johannesburg/33/94
Reference Antigen
  A/Shangdong/09/93        640                 320                   160
  A/Guangdong/25/93        320                1280                  1280
  A/Johannesburg/33/94     160                1280                  1280
Recent Isolates
  A/Alaska/06/95           160                1280                   640
  A/Washington/02/95       160                 640                   640
  A/Korea/10/95            160                 640                  1280
  A/Netherlands/01/95      160                 640                   640
  A/Canada/20/95           160                 640                   640
* A fourfold difference in hemagglutination-inhibition titers with two viruses is usually
  indicative of antigenic variation between viruses.

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Note: To print large tables and graphs users may have to change their printer settings to landscape and use a small font size.

TABLE 2. Hemagglutination-inhibition titers of influenza B viruses with serum specimens from infected
ferrets *
                                              Ferret Antiserum
Viral Antigen         B/Panama/45/90   B/Beijing/184/93   B/Shanghai/04/94   B/Harbin/07/94
Reference Antigen
  B/Panama/45/90            640              320                320               320
  B/Beijing/184/93          160              320                320               320
  B/Shanghai/04/94          160              320                320               640
  B/Harbin/07/94            160              320                320               640
Recent Isolates
  B/Pennsylvania/05/95      160              320                320               640
  B/Iowa/01/95               80              320                160               320
  B/England/73/94           160              320                320               640
  B/Canada/01/95            160              640                320               320
* A fourfold difference in hemagglutination-inhibition titers with two viruses is usually indicative
  of antigenic variation between viruses.

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