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

Laboratory tests with animal sera have demonstrated that influenza A(H1N1) viruses isolated from outbreaks in Southeast Asia and some parts of Oceania from approximately March to August 1983 (1) have undergone antigenic drift from the previously prevalent strains. Post-infection ferret serum to A/Brazil/11/78, which reacted well with the A/England/333/80-like strains that had been preponderant among H1N1 isolates during the past 2-3 years (2,3), reacts poorly with most recent isolates in hemagglutination inhibition (HI) tests. Ferret antiserum to another variant, A/India/6263/80, which has cocirculated with A/England/333/80 (2,3), also reacts poorly with the recent isolates.

When compared in reciprocal HI tests, the recent isolates are heterogenous (Table 1). One type of reaction pattern is exemplified by the virus A/Chile/1/83, which is rather poorly inhibited by all heterologous sera. The A/Chile/1/83 virus itself induces broadly reactive antibodies when used to infect ferrets. Viruses with similar characteristics to A/Chile/1/83 have comprised the majority of recently tested isolates from Singapore and Thailand. Viruses from New Caledonia and New Zealand, however, have exhibited two alternative types of reaction patterns. Examples are shown with A/New Caledonia/4/83 and A/Dunedin/7/83 (Table 1). When inoculated into ferrets, each virus induces antibody that reacts more with itself than with other 1983 variants. Both A/New Caledonia/4/83- and A/Dunedin/7/83-like variants have been found from within the same outbreaks in New Caledonia and New Zealand. Preliminary tests in man show that vaccine containing A/Brazil/11/78 virus induces antibody that inhibits the new H1N1 variants better than does post-infection A/Brazil/11/78 ferret serum. Reported by WHO Collaborating Center for Influenza, Influenza Br, Div of Viral Diseases, Center for Infectious Diseases, CDC.

Editorial Note

Editorial Note: It is possible that the genetic difference between the new variants from Oceania is very small and that the two strains shown represent minor variants in a common evolutionary path; this would parallel a situation observed with some cocirculating variants of influenza A(H3N2), such as A/Bangkok/1/79 and A/Bangkok/2/79, whose hemagglutinin antigenic sites differ from each other by only two amino acids (4).

Continuing study of influenza A(H1N1) viruses from different parts of the world, including the Northern Hemisphere, is necessary to determine whether any of the new variants will predominate. Despite the antigenic differences detected with highly specific, post-infection ferret sera, it is likely that this year's vaccine, which contains A/Brazil/11/78 as its H1N1 antigenic component, would be of benefit in protecting many people against the new virus variants if they spread in the United States, due to the broader antibody response in man. Furthermore, H3N2 and type B viruses, which continue to circulate in the world, are also included in the vaccine.

References

1. CDC. Influenza--worldwide. MMWR 1983;32:502-4.

2. Cox NJ, Bai ZS, Kendal AP. Laboratory-based surveillance of influenza A(H1N1) and A(H3N2) viruses in 1980-81: antigenic and genomic analyses. Bull WHO, 1983;143-52.

3. World Health Organization. Influenza in the world, October 1981-September 1982. WHO Wkly Epidem Rec 1982;50:389-93.

4. Both GW, Sleigh MS, Cox NJ, Kendal AP. H3 influenza haemagglutinin from 1968-1980: multiple evolutionary path ways and sequential amino acid changes at key antigenic sites. J Virol 1983;48:52-60.

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