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Update: Influenza Activity -- United States and Worldwide, and Composition of the 1993-94 Influenza Vaccine

In collaboration with the World Health Organization (WHO) international 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 1992-93 season and describes the composition of the 1993-94 influenza vaccine. United States

During the 1992-93 influenza season, influenza activity in the United States began in October and increased gradually from December through late February. Recent reports suggest that activity may be declining in some areas. The number of isolates and the ratio of specimens positive for influenza to total specimens submitted for respiratory virus testing declined slightly during late February and early March. Weekly reports by state and territorial epidemiologists indicated increasing levels of influenza-like illness (ILI) from December through late February and a slight decline from late February through early March.

From October through January, influenza B viruses predominated and outbreaks were reported primarily among school-aged persons; outbreak activity reported among older adults was limited, and no excess occurred in influenza-associated mortality. Recent increased circulation of influenza A(H3N2) viruses has been associated with reports of increasing numbers of culture-confirmed outbreaks in nursing homes and other chronic-care facilities.

From September 27, 1992, through March 6, 1993, 1791 (86%) of the 2087 influenza virus isolates reported by the WHO collaborating laboratories in the United States were influenza type B. Influenza B viruses isolated in the United States this season have been antigenically similar to the B/Panama/45/90 virus included in the 1992-93 influenza vaccine. However, the proportion of influenza type A viruses has steadily increased since mid-January. From September 27, 1992, through January 16, 1993, 10 (2%) of the 578 influenza viruses reported were influenza type A compared with 144 (14%) of the 1026 viruses reported for January 17 through February 13 and 142 (29%) of the 483 viruses reported for February 14 through March 6. Of the 296 influenza A viruses isolated, 22 (7%) were subtyped as A(H1N1) and 115 (39%) as A(H3N2); 159 (54%) have not yet been subtyped. Of the influenza A(H3N2) viruses isolated in the United States this season and characterized at CDC, six were antigenically similar to the vaccine strain A/Beijing/353/89, and 28 were similar to the antigenic variant A/Beijing/32/92 (Table 1).

The proportion of deaths associated with pneumonia and influenza to total deaths reported through CDC's 121-city mortality reporting system exceeded the epidemic threshold for 1 week (ending February 20) but remained below the epidemic threshold for the following 2 weeks. Worldwide

Influenza activity worldwide has occurred at moderate levels during the 1992-93 season. Influenza viruses have been isolated in association with sporadic activity and outbreaks in Asia, Europe, and North America. Although most activity has been associated with influenza B viruses, influenza A(H3N2) viruses were also isolated during periods of sporadic activity or outbreaks in 21 countries. Isolation of influenza A(H1N1) viruses has been rare.

Influenza B viruses were first reported in France, Japan, and the United States during October 1992 and predominated in all countries reporting influenza during the first months of the season. They remain the most common and widespread viruses isolated in Europe and North America. Influenza B viruses have been isolated in association with outbreaks among schoolchildren in China, Hungary, Japan, Sweden, the United Kingdom, and the United States. Other countries reporting isolation of influenza B viruses include Belgium, Bulgaria, Canada, Croatia, the Czech Republic, Denmark, Finland, France, Germany, Hong Kong, Israel, Italy, Lithuania, the Netherlands, Norway, Portugal, Romania, the Russian Federation, Singapore, the Slovak Republic, Spain, Switzerland, Taiwan, and Thailand.

Although influenza A(H3N2) viruses have been isolated less frequently worldwide, they were first reported in November 1992 during sporadic activity or small outbreaks in Japan, Sweden, and the United States. Japan subsequently reported culture-confirmed widespread outbreaks during December 1992 and January and February 1993. Influenza A(H3N2) viruses were isolated during outbreaks in northern China during late December and January. As of late February, influenza A(H3N2) viruses had also been isolated in Belgium, Bulgaria, Canada, Croatia, the Czech Republic, Finland, France, Germany, Indonesia, Italy, the Netherlands, Norway, Romania, the Russian Federation, Singapore, Spain, and the United Kingdom.

Influenza A(H1N1) viruses have been isolated during periods of sporadic activity in Canada, France, the Netherlands, the United Kingdom, and the United States. Composition of the 1993-94 Vaccine

For the 1993-94 influenza season, the Food and Drug Administration Vaccines and Related Biologicals Advisory Committee (VRBAC) has recommended that the trivalent influenza vaccine for the United States contain A/Texas/36/91-like(H1N1), A/Beijing/32/92-like(H3N2), and B/Panama/45/90-like viruses. This recommendation was based on the antigenic analysis of recently isolated influenza viruses, the patterns of spread of antigenic variants, and the antibody response of persons previously vaccinated with the 1992-93 influenza vaccine.

More than 300 influenza B viruses isolated worldwide since October 1992 have been characterized antigenically. All are similar to the B/Panama/45/90 vaccine strain, and to the closely related variant B/Qingdao/102/91 (1). Vaccines containing B/Panama/45/90-like viruses induced antibodies with similar frequency and titer to the vaccine virus and to representative recent isolates. Therefore, for the 1993-94 vaccine, the VRBAC recommended retaining the current B/Panama/45/90-like vaccine strain.

Although viruses similar to the A/Beijing/353/89 vaccine strain continue to be isolated, antigenic analysis of influenza A(H3N2) viruses indicates that many recently isolated strains from Asia, Europe, and North America are similar to the antigenic variant A/Beijing/32/92 (Table 1). Vaccines containing A/Beijing/353/89-like antigen induced a good response to this vaccine strain. In contrast, this vaccine induced lower and less frequent antibody responses to recent A(H3N2) isolates, such as A/Beijing/32/92, than to the A/Beijing/353/89 vaccine strain (Table 2). Therefore, the VRBAC recommended changing the influenza A(H3N2) vaccine component to an A/Beijing/32/92-like strain for the 1993-94 season.

Although the number of isolates of influenza A(H1N1) viruses has been limited, all those characterized have been closely related to the reference strains A/Taiwan/1/86 or A/Texas/36/91 (2). Antibody induced by vaccination with the A/Texas/36/91 vaccine component induced good immune responses to the vaccine strain and to representative recent isolates. Thus, the VRBAC recommended retaining the A/Texas/36/91-like vaccine strain for the 1993-94 vaccine.

Reported by: M Chakraverty, PhD, Central Public Health Laboratory, J Skehel, 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, Commonwealth Serum Laboratories, Parkville, Australia. K Nerome, PhD, National Institute of Health, Tokyo. J Groothuis, MD, Univ of Colorado School of Medicine, P Graves, G Meiklejohn, MD, Univ of Colorado Medical Center, Denver. A Biache, MSN, Goodwin House, Inc, Alexandria, Virginia. P Gross, MD, Hackensack Medical Center, New Jersey. WHO National Influenza Centers, Microbiology and Immunology Support Svcs, World Health Organization, Geneva. Participating state and territorial health department epidemiologists and state public health laboratory directors. Div of Virology, Center for Biologics Evaluation and Research, Food and Drug Administration. Epidemiology Activity, and WHO Collaborating Center for Surveillance, Epidemiology, and Control of Influenza, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: The recent increase in influenza A activity in the United States indicates a continuing need for surveillance, including culture of specimens from patients with ILI. Although the severity and types of future influenza epidemics cannot be predicted reliably, the recent increased isolation of variant type A(H3N2) viruses suggests that such viruses may predominate during the 1993-94 influenza season.

Strains to be included in the influenza vaccine for the United States are selected from January through March each year to meet the production schedule required for the manufacture, quality control, and distribution of the more than 40 million doses of vaccine before the next influenza season. Specific recommendations for the use of the newly constituted influenza vaccine will be made by the Public Health Service Advisory Committee on Immunization Practices and published in the MMWR Recommendations and Reports during May 1993.


  1. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1993-94 season. Wkly Epidemiol Rec 1993;68:57-60.

  2. CDC. Update: influenza activity -- United States, 1991-92 season. MMWR 1992;41:63-5.

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