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Current Trends Antigenic Variation of Recent Influenza A(H1N1) Viruses

Antigenic analysis of recent influenza A(H1N1) viruses, including those reported from several countries in Southeast Asia (1), has detected antigenic drift from the previously prevalent strain related to A/Chile/1/83(H1N1). The antigenic variation has been seen both in reciprocal hemagglutination-inhibition tests using animal antisera and in comparisons of antibody levels against the newer variants in persons immunized with vaccine containing A/Chile/1/83 antigen.

Animal antiserum to A/Chile/1/83 is less effective in inhibiting the newer A(H1N1) variants than in inhibiting A/Chile/1/83-like viruses, although the degree of reactivity varies among isolates of the newer A(H1N1) variants (Table 3). In contrast, antisera prepared against representative new isolates (e.g., A/Singapore/6/86, A/Taiwan/1/86) react well with all other recent isolates but extremely poorly with A/Chile/1/83. In tests with animal antisera, the new variants also differed from other variants that have cocirculated with A/Chile/1/83 since 1983-1984 (e.g., A/Victoria/7/83, A/Dunedin/27/83).

Studies of antibody response in recipients of last year's influenza vaccine containing A/Chile/1/83 antigen demonstrate the difference of the new variants (Table 4). Approximately 80% of adult vaccinees developed titers to A/Chile/1/83 of 160 or higher and had a postvaccination geometric mean titer (GMT) of 320. In contrast, only 15%-30% of the vaccinees developed the same titers to the new variants, and the postvaccination GMT was about sixfold lower. However, since postvaccination titers of 40 or greater have been associated with reduced influenza infection and illness, it is possible that the A/Chile/1/83 antigen in the 1986-1987 trivalent vaccine will provide at least partial protection against the new variants.

Through July 1986, influenza A(H1N1) variants had been detected among virus isolates from Hong Kong, Malaysia, and New Zealand; from an imported case in England believed to have originated in India; and among viruses from a few outbreaks and sporadic cases of influenza A(H1N1) that occurred in Japan last winter. The presence of an A(H1N1) variant has also been reported from the Soviet Union, where last winter's epidemic was primarily caused by influenza B and influenza A(H3N2) viruses. Recently, influenza A viruses, some confirmed as A(H1N1) strains, have also been reported in Australia, Thailand, American Samoa, and Palau, Micronesia. Influenza outbreaks in Palau and elsewhere in the U.S. Pacific Trust Territories are presently being investigated. Reported by National Influenza Centers in collaboration with the World Health Organization, Geneva; WHO Collaborating Centre for Influenza, Influenza Br, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: After the World Health Organization (WHO) Collaborating Centres for Influenza detected variants of influenza A(H1N1) viruses among isolates submitted for reference analysis from influenza outbreaks in the People's Republic of China, Malaysia, Singapore, and Taiwan, data were reported indicating that related strains had been detected at the end of an influenza A(H3N2) epidemic in Japan last winter. The most recent reports suggest that the variant may have been introduced into other countries widely separated in the Pacific Basin from Southeast Asia (e.g., Australia, New Zealand, and several islands in the U.S. Pacific Trust Territories) and possibly India. Reports of outbreaks in Southeast Asia indicate primarily children and young adults have been affected, suggesting that the general epidemiologic characteristics of the new variant are similar to those of other type A(H1N1) variants occurring since 1977.

Based on the available reports, WHO has suggested that national health authorities consider the addition of the new type A(H1N1) virus to other strains already recommended for incorporation in 1986-1987 influenza vaccines, either as an extra component in current trivalent vaccines or as a monovalent vaccine (2). In the United States, the vaccine production schedule permits manufacture only of a supplemental monovalent vaccine with a new type A(H1N1) virus. After consideration of available data, the U.S. Public Health Service is recommending production and use of such a vaccine to maximize protection against the new variant, particularly for high-risk children and young adults who appear to be most susceptible to illness caused by current type A(H1N1) viruses (3). If outbreaks of influenza-like illnesses occur in the United States early this fall, prompt laboratory diagnosis will be necessary to establish whether they are due to influenza A viruses and to guide appropriate use of the available antiviral agent, amantadine hydrochloride (4).


  1. CDC. Update: influenza activity--worldwide. MMWR 1986;35:433-4.

  2. World Health Organization. Composition of influenza virus vaccines for use in the 1986-87 season: an update. Wkly Epidem Rec 1986;61:237-8.

  3. ACIP. Monovalent influenza A(H1N1) vaccine, 1986-1987. MMWR 1986;35:517-21.

  4. ACIP. Prevention and control of influenza. MMWR 1986;35:317-26, 331.

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