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Current Trends Update: Influenza Activity -- United States and Worldwide, and the Composition of the 1991-92 Influenza Vaccine

United States

During the 1990-91 influenza season, influenza activity in the United States was at relatively low levels, as evidenced by the small number of reported outbreaks in institutions (primarily schools) and the lack of substantial mortality from pneumonia and influenza.

In mid-November, state epidemiologists first reported sporadic* influenza activity in the mid-Atlantic and New England regions. By mid-January, widespread influenza activity was reported in New York; regional activity was reported in six states east of the Mississippi River, Missouri, and Nebraska. Influenza activity in the United States peaked from February 10 through March 2, when eight to 10 states (predominantly from the four central regions of the country) reported widespread activity each week. After March 9, four states in the Mountain or Pacific regions reported widespread activity.

*Levels of activity are: 1) Sporadic--sporadically occurring influenza-like illness or culture-confirmed influenza, with no outbreaks detected; 2) Regional--outbreaks of influenza-like illness or culture-confirmed influenza in counties having a combined population of less than 50% of the state's total population; 3) Widespread--outbreaks of influenza-like illness or culture-confirmed influenza in counties having a combined population of greater than or equal to 50% of the state's total population.

Of the greater than 2500 influenza virus isolates identified and reported to CDC this season, the majority (93%) were influenza type B. Almost all of an antigenically tested sample of these were related to influenza B/Yamagata/16/88, which was included in this year's vaccine, but were antigenically closer to B/Panama/45/90, a minor variant.

Influenza A viruses constituted 0-2% of the influenza isolates reported from most regions but were approximately 20% of the isolates from the Mountain and Pacific regions. Although sporadic reports of influenza A occurred throughout the season, since mid-February a slight increase in both A(H1N1) and A(H3N2) has occurred. In some areas, the increase was associated with a reported recrudescence of influenza-like illness. As of March 31, 64 (54%) of the 118 reported influenza A viruses with known subtype were A(H3N2) strains, and 54 (46%) were A(H1N1) strains. Of the 11 A(H1N1) strains antigenically tested at CDC, all closely resembled A/Taiwan/1/86, the 1990-91 vaccine strain. Of 16 A(H3N2) strains tested, all showed variation from the 1990-91 vaccine strain, A/Shanghai/16/89; eight were closely related to the reference strain A/Beijing/353/89. Antiserum to A/Beijing/353/89 also reacted well with most of the other recent isolates.


During the 1990-91 season, influenza activity worldwide occurred at low levels. Although influenza B, influenza A(H1N1), and influenza A(H3N2) viruses were all associated with relatively small local or regional outbreaks in different areas of the world, major epidemics affecting entire countries were not reported.

In Canada and Europe, as in the United States, the predominant circulating virus was influenza type B. Although outbreaks occurred in many locations, they were usually local and occurred primarily among school children. Most influenza B isolates were related to B/Yamagata/16/88 and B/Panama/35/90, except in the United Kingdom where viruses closely related to the previously prevalent strain, B/Victoria/2/87, were a substantial proportion of influenza B isolates.

Viruses closely related to A/Taiwan/1/86(H1N1) have been isolated sporadically, but local outbreaks associated with these viruses have been identified only in Stockholm during December and in northern Hungary during late February and early March. Sporadic cases have occurred in Canada, Korea, Japan, and countries in Europe.

In Europe during the 1990-91 season, influenza activity associated with A(H3N2) viruses was less than that associated with A(H1N1) viruses. Sporadic isolates were reported in Canada, Switzerland, the Union of Soviet Socialist Republics, Egypt, and Thailand. Outbreaks were reported in Korea and Japan. Detailed antigenic analyses of recent strains of A(H3N2) viruses are in progress.

Composition of the 1991-92 Influenza Vaccine

For the 1991-92 season, the Food and Drug Administration (FDA) Vaccine Advisory Panel has recommended that the trivalent influenza vaccine contain A/Taiwan/1/86like(H1N1), A/Beijing/353/89-like(H3N2), and B/Panama/45/90-like viruses. This recommendation was based on the antigenic analysis of recent isolates and studies of the antibody response of persons previously vaccinated with the 1990-91 influenza vaccine.

Antigenic analysis of influenza A(H1N1) isolates from outbreaks in Stockholm and from sporadic isolates from the United States indicates that drift from the 1990-91 vaccine strain (A/Taiwan/1/86) has not occurred. Furthermore, antibody induced by this vaccine component reacts well with currently circulating strains.

From May through September 1990, influenza A(H3N2) was active in the southern hemisphere and isolates were predominately similar to a new variant A/Beijing/353/89 previously detected only in northern China (Table 1) (1). During the 1990-91 influenza season, a limited number of isolates available for testing from North America and Japan were more closely related to A/Beijing/353/89 than to the 1990-91 vaccine strain, A/Shanghai/16/89(H3N2) (2). In persons who received the 1990-91 vaccine containing A/Shanghai/16/89 as the A(H3N2) component, the postvaccine geometric mean titer to A/Beijing/353/89 was approximately 50% of that to the vaccine strain, A/Shanghai/16/89. Thus, the World Health Organization (WHO) (3) and the FDA Vaccine Advisory Panel recommended changing the A(H3N2) strain for the 1991-92 season from A/Shanghai/16/89 to A/Beijing/353/89.

Two strains of influenza B, B/Victoria/2/87 and B/Yamagata/16/88, have cocirculated in the world since 1988. Both viruses circulated in Europe during the 1989-90 epidemic season, and in Australia and Africa during the southern hemisphere epidemic season. Since July 1990, the number of B/Victoria/2/87-like viruses isolated in Asia, the Americas, and Europe (except for the United Kingdom) has been small. Antigenic heterogeneity of viruses related to B/Yamagata/16/88 was detected among isolates during the past year, and the B/Panama/45/90 virus is more representative of the majority of recent isolates (Table 2). Although antibody induced in adult vaccinees by the B/Yamagata/16/88 vaccine component is broadly reactive against B/Panama/45/90, children may have geometric mean titers twofold or threefold lower against the B/Panama/45/90-like viruses than against B/Yamagata/16/88-like viruses (Table 3). Therefore, for the 1991-92 vaccine, the FDA vaccine advisory panel recommended changing the influenza B vaccine component from B/Yamagata/16/88 to B/Panama/45/90.

Reported by: M Grandien, PhD, National Bacteriological Laboratory, Stockholm, Sweden. K Nerome, PhD, National Institute of Health, Tokyo, Japan. M Chakraverty, PhD, Central Public Health Laboratory, London; G Schild, PhD, J Wood, PhD, National Institute for Biological Standards and Control, Hertfordshire; J Skehel, PhD, National Institute for Medical Research, London, United Kingdom. P Palmer, K Edwards, MD, Vanderbilt Univ, Nashville, Tennessee. P Graves, G Meiklejohn, MD, Univ of Colorado, Denver, Colorado. F Ruben, MD, Univ of Pittsburgh, Pittsburgh, Pennsylvania. WHO National Influenza Centers, Microbiology and Immunology Support Svcs, World Health Organization, Geneva, Switzerland. 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 the WHO Collaborating Center for Influenza, Influenza Br, Div of Viral and Rickettsial Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note:

The 1990-91 influenza season in the United States and many other countries was characterized by a predominance of influenza B among circulating strains and limited mortality. The increase in influenza A activity in the United States late this season indicates a continuing need for surveillance, including culture of specimens from patients with influenza-like illness. Although the severity and types of future influenza epidemics cannot be reliably predicted, the increased isolation of type A viruses, including A(H3N2) strains, suggests that such viruses may predominate next winter.

The composition of influenza vaccine for the United States is determined between January and late March each year to meet the production schedule required for greater than 30 million doses to be manufactured, quality controlled, and distributed before onset of the next influenza season. As in the past, 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 an MMWR Recommendations and Reports.


  1. CDC. Update: influenza activity--worldwide and recommendations for influenza vaccine composition for the 1990-91 influenza season. MMWR 1990;39:293-6.

  2. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1990-1991 season. Wkly Epidemiol Rec 1990;65:53-6.

  3. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1991-1992 season. Wkly Epidemiol Rec 1991;66:57-60.

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