Update on Influenza Activity -- United States and Worldwide, with Recommendations for Influenza Vaccine Composition for the 1988-89 Season

Worldwide

Although influenza activity in the United States this season has been primarily associated with type A(H3N2), influenza B has been the predominant virus type reported from other areas of the world.

Between October 1987 and April 1988, localized outbreaks of influenza B occurred in Finland, France, Greece, the United Kingdom, West Germany, and the Union of Soviet Socialist Republics (U.S.S.R.). In Japan, a localized outbreak of influenza B occurred during November of 1987, and sporadically occurring cases were confirmed through February of 1988. Sporadically occurring cases of influenza B were also confirmed from North Korea during March. Influenza B has been the most frequently isolated influenza virus in the western provinces of Canada; during February, it was associated with an outbreak in Calgary, Alberta. Influenza B has also been the predominant virus type in Ontario since October of 1987. In the United States, influenza B has accounted for 9% of isolates reported nationally by World Health Organization (WHO) Collaborating Laboratories; only Hawaii has reported influenza B as the predominant virus type.

Influenza A(H3N2) caused localized outbreaks in Taiwan from September through November of 1987. Singapore reported isolating influenza A(H3N2) viruses from sporadically occurring cases during September and October 1987, and Japan made similar reports from October 1987 through February 1988. In Europe, influenza A(H3N2) was associated with localized outbreaks in East Germany and Romania during March. Localized outbreaks of influenza A(H3N2) in the U.S.S.R. during January and February escalated to widespread activity during March, as an epidemic of influenza B was waning. Sporadically occurring cases of influenza A(H3N2) were confirmed in several European countries, including Finland, France, Hungary, Norway, and the United Kingdom between January and March 1988 and in Egypt during January and February. Sporadic influenza A(H3N2) activity has also been reported in several Canadian provinces, and a few localized outbreaks have been associated with influenza A, but the subtype of these viruses has not been identified.

Sporadically occurring cases of influenza A(H1N1) have been confirmed in the United States since January of this year and have been confirmed recently in Canada. In Europe, influenza A(H1N1) was isolated from sporadically occurring cases in Switzerland during February and March. A localized outbreak of influenza A(H1N1) was reported in a primary school in Italy during March.

United States

Surveillance indicators in the United States suggest that influenza activity is waning. Reports from state and territorial epidemiologists have shown a progressive decline in outbreak activity since the week ending March 12, when 57% of the states were still reporting regional or widespread outbreaks of influenza-like illness. For the week ending April 16, two states reported widespread activity, and five states reported regional activity. For the same week, the percentage of patients visiting reporting sentinel physicians for influenza-like illnesses dropped to a low of 3.4%, from a peak of 8.1% for the week ending February 20. The number of specimens tested and the number of influenza viruses isolated at WHO Collaborating Laboratories have also declined since the end of February, from a peak of over 1,700 specimens tested with approximately 300 influenza viruses isolated, to 448 specimens tested and 43 viruses isolated for the week ending April 16. However, the ratio of pneumonia and influenza deaths to deaths from all causes, which has declined since reaching a peak on the week ending March 5, remains above the epidemic threshold for the ninth week.

Antigenic Analysis of Recent Influenza Isolates and Recommendations for Influenza Vaccine Composition for the 1988-89 Season

As previously reported (1), influenza A(H3N2) viruses isolated in the United States and in other parts of the world during the 1987-88 influenza season were found to be antigenically distinct from viruses that circulated from 1985 through the spring of 1987. Although influenza B viruses have been isolated less frequently, it has become clear, as more isolates become available, that antigenic variation has also occurred among these viruses. Analysis of recent influenza B virus isolates indicates that these antigenic variants are different from the previously prevalent strains B/USSR/100/83 and B/Ann Arbor/1/86 (Table 1). Most recent isolates resemble the reference strain B/Victoria/2/87. The additional antigenic variant B/USSR/2/87, which was isolated in Moscow in December 1987, has been identified less frequently than strains that resemble B/Victoria/2/87.

The antibody response induced by the current type B vaccine strain, B/Ann Arbor/1/86, is greater to the homologous virus than to the reference variant B/Victoria/2/87 (Table 2). Vaccinees in all age groups developed neutralizing antibody titers greater than or equal to100 more frequently to B/Ann Arbor/1/86 than to the B/Victoria/2/87 variant (2), and the geometric mean titers were higher to the homologous vaccine component than to the B/Victoria/2/87 variant. During the 1987-88 season, influenza A(H1N1) viruses have continued to resemble the A/Taiwan/1/86 and A/Singapore/6/86 viruses, which were first isolated in Asia in 1986.

Based on antigenic analysis of recent influenza viruses, WHO has recommended updated type A(H3N2) and type B antigens for influenza vaccines for use during the 1988-89 influenza season. WHO recommends the same A(H1N1) component that was used in the 1987-88 vaccine (3). Consistent with these recommendations, the Public Health Service has recommended the following antigens for the trivalent influenza vaccine to be manufactured in the United States for the 1988-89 influenza season: A/Taiwan/1/86(H1N1), A/Sichuan/2/87(H3N2), and B/Victoria/2/87. Reported by: F Ruben, MD, Univ of Pittsburgh, Pittsburgh, Pennsylvania. K Edwards, MD, P Palmer, Vanderbilt Univ, Nashville, Tennessee. RB Couch, MD, WA Keitel, MD, Baylor Coll of Medicine, Houston, Texas. National Influenza Centers, Microbiology and Immunology Support Svcs, WHO, Geneva. Div of Virology, Office of Biologics, FDA. Participating State and Territorial Epidemiologists and State Laboratory Directors. WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC. References

1. Centers for Disease Control. Influenza--United States. MMWR 1988;37:207-9.

2. Harmon MW, Rota PA, Walls HH, Kendal AP. Antibody response in humans to influenza virus type B host-cell-derived variants after vaccination with standard (egg-derived) vaccine or natural infection. J Clin Microbiol 1988;26:333-7.

3. World Health Organization. Recommended composition of influenza virus vaccines for use in the 1988-89 season. Wkly Epidem Rec 1988;9:57-60.

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