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Update: Influenza Activity -- United States and Worldwide, and Composition of the 1992-93 Influenza Vaccine

During the 11 influenza seasons from 1977 through 1988, more than 10,000 excess deaths attributed to pneumonia and influenza (P&I) were reported during each of seven seasons, and approximately 45,000 deaths were reported during each of two seasons (CDC, unpublished data, 1992). The most important strategy for preventing influenza-associated morbidity and mortality is vaccination of persons in high-risk groups with vaccine closely matched to circulating strains. In collaboration with state and local health departments, CDC conducts surveillance to monitor influenza activity and to detect antigenic changes in the circulating strains of influenza virus. This report summarizes surveillance for influenza in the United States and worldwide during the 1991-92 season and describes the composition of the 1992-93 influenza vaccine. United States

During the 1991-92 influenza season, substantial influenza activity began in October and peaked from December 1 through January 18 when 11-20 states reported widespread activity * each week. From January 19 through March 21, activity steadily declined; for the week ending March 21, no states reported widespread activity, and three states reported regional activity.

Before mid-December, outbreaks were reported primarily in schools; subsequently, outbreaks were reported in schools, nursing homes, and other institutional settings. Based on data from death certificates provided by CDC's 121 U.S. Cities Mortality Surveillance System, excess mortality was associated with P&I. The proportion of deaths attributable to P&I exceeded the epidemic threshold for 7 consecutive weeks (from the weeks ending December 28 through February 8) and peaked at 7.9% of all deaths during the week ending January 18.

Of the 5861 influenza virus isolates reported to CDC, more than 99% were influenza A. Of the influenza A virus isolates subtyped, 81% were influenza A(H3N2), and 19% were influenza A(H1N1). As of April 10, 179 (99%) of the 180 influenza A(H3N2) viruses characterized at CDC resembled A/Beijing/353/89, the A(H3N2) component included in the 1991-92 influenza vaccine. Influenza A(H1N1) viruses were isolated most frequently in the mid-Atlantic and South Atlantic regions and accounted for 30% and 52% of influenza A isolates, respectively. Influenza A(H1N1) viruses were characterized by moderate antigenic heterogeneity. Worldwide

Influenza activity worldwide occurred at moderate levels during the 1991-92 influenza season. Although most activity was associated with influenza A(H3N2), influenza A(H1N1) viruses were isolated in 17 countries; influenza B viruses were isolated rarely. Most countries reported that influenza activity began in December and peaked in late January or early February.

In Europe, influenza A(H3N2) virus was the predominant isolate in the Commonwealth of Independent States, Czechoslovakia, Denmark, Finland, France, Greece, Hungary, Italy, the Netherlands, Norway, Portugal, Romania, Spain, Sweden, Switzerland, and the United Kingdom; in some of these countries, it was associated with localized outbreaks. In Asia, outbreaks of influenza A(H3N2) were reported in Japan, Korea, and the People's Republic of China.

Although influenza A(H1N1) viruses were isolated less frequently worldwide, Belgium and Japan reported that A(H1N1) viruses constituted the majority of isolates and were isolated from outbreaks. France, Germany, the Netherlands, and the United Kingdom reported that approximately 20% of the influenza A viruses isolated were A(H1N1). In these countries, the proportion of influenza A(H1N1) viruses isolated increased during the latter part of the season as influenza A(H3N2) activity declined. Bulgaria, Canada, the Commonwealth of Independent States, Croatia, Finland, Norway, Spain, Switzerland, and Yugoslavia reported only sporadic cases of influenza A(H1N1).

Outbreaks caused by influenza B viruses were reported in Greece, Korea, the People's Republic of China, Taiwan, Tunisia, and Yugoslavia. Sporadic cases were reported from Canada, the Commonwealth of Independent States, Finland, France, Norway, Poland, Sweden, the United Kingdom, and the United States. Composition of the 1992-93 Influenza Vaccine

For the 1992-93 influenza season, the Food and Drug Administration (FDA) Vaccines and Related Biologicals Advisory Committee (VRBAC) has recommended that the trivalent influenza vaccine contain A/Texas/36/91-like(H1N1), A/Beijing/353/89-like(H3N2), and B/Panama/45/90-like viruses. This recommendation was based on the antigenic analyses of recent isolates and studies of the antibody response of persons previously vaccinated with the 1991-92 influenza vaccine.

Antigenic analyses of influenza A(H1N1) isolates from North America and Europe indicate that antigenic heterogeneity exists among recent isolates (1,2). Approximately 40% of isolates, represented by the A/Texas/36/91 strain, exhibit drift from the A/Taiwan/1/86 vaccine strain. Antibody induced by the A/Taiwan/1/86 vaccine component reacted at lower titers with the A/Texas/36/91 virus and other representative 1991-92 A(H1N1) viruses than with A/Taiwan/1/86 in several vaccine studies (Table 1). Therefore, the VRBAC recommended changing the influenza A(H1N1) vaccine component to an A/Texas/36/91-like strain for the 1992-93 vaccine.

Although some antigenic heterogeneity exists among influenza A(H3N2) isolates, most viruses isolated throughout the world were antigenically related to the A/Beijing/353/89 vaccine strain (2). Antibody induced by this vaccine component reacted similarly with recent isolates, such as the A/Washington/15/91 virus, in serum from all age groups, and postvaccination geometric mean titers (GMTs) of antibody to the recent isolates were approximately 50%-100% of those to the vaccine virus. Thus, the World Health Organization and the VRBAC recommended retaining the A/Beijing/353/89 vaccine strain.

Two distinct strains of influenza B virus, related to either the B/Victoria/2/87 or the B/Yamagata/16/88 and B/Panama/45/90 reference strains, have cocirculated in the world since 1988. Since October 1991, only two strains related to the B/Victoria/2/87 reference strain have been identified worldwide. Although relatively few influenza B viruses related to the B/Yamagata/16/88 and B/Panama/45/90 reference strains have been isolated, antigenic heterogeneity has been observed among them. Postvaccination serum specimens obtained from vaccinees of all age groups reacted well with the vaccine virus B/Panama/45/90, and except for serum from the youngest children, also reacted well with B/Victoria/2/87. In all age groups, the postvaccination GMTs to recent influenza B isolates were approximately 50%-100% of those for the vaccine virus. Therefore, for the 1992-93 vaccine, the VRBAC recommended retaining the B/Panama/45/90 vaccine strain.

Reported by: K Nerome, PhD, National Institute of Health, Tokyo, Japan. 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. J Groothuis, MD, Univ of Colorado School of Medicine and The Children's Hospital; P Graves, G Meiklejohn, MD, Univ of Colorado Medical Center, Denver. A Biache, MSN, Goodwin House, Inc., Alexandria, Virginia. 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 Influenza, Div of Viral and Rickettsial Diseases, National Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: Strains to be included in the influenza vaccine for the United States are selected from late January through March each year to meet the production schedule required for the manufacture, quality control, and distribution of more than 30 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 Immunization Practices Advisory Committee of the Public Health Service and published in the MMWR Recommendations and Reports dated May 15, 1992.


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

  2. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1992-1993 season. Wkly Epidemiol Rec 1992;9:57-60.

    • 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 population; 3) widespread -- outbreaks of ILI or culture-confirmed influenza in counties having a combined population of 50% or more of the state's total population.

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