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Current Trends Antigenic Analysis of Recent Influenza Isolates

Influenza type A(H1N1) and type B viruses received at CDC thus far this winter from outbreaks and sporadic cases in the United States have been closely related to the reference strains A/England/333/80(H1N1) and B Singapore/222/79, respectively. Since about July 1981, influenza type A(H3N2) viruses have been received from Australia, Chile, Guam, Indonesia, Japan, People's Republic of China, Taiwan Province of China, and Trinidad and Tobago. As in the preceding year, the isolates have exhibited heterogeneous reaction patterns in hemagglutination-inhibition tests with ferret serum specimens. Varying proportions of the viruses from different locations are more closely related to A/Texas/1/77 or A/Bangkok/1/79. A minority of recent isolates have been found to exhibit some further antigenic drift from earlier strains and to resemble the virus A/Shanghai/31/80 isolated in December 1980 from a sporadic case of influenza. As shown in Table 1, A/Shanghai/31/80 exhibits an asymmetric antigenic difference from A/Bangkok/1/79, in that antiserum to A/Bangkok/1/79 usually inhibits the variant to a titer 4-fold lower than homologous, whereas antiserum to A/Shanghai/31/80 reacts almost equivalently with itself and with A/Bangkok/1/79. A further characteristic of A/Shanghai/31/80-like viruses is their low inhibition by A/Texas/1/77 and A/Bangkok/2/79 antisera. The above-described variants have been isolated concurrently, and there is no clear evidence of A/Shanghai/31/80-like viruses, for example, achieving predominance and being responsible for major outbreaks or epidemics in Asia or elsewhere. Prevalence of antibody to A/Bangkok/1/79 and A/Shanghai/31/80 appears similar in the general population in the United States and the United Kingdom, where this has been studied by the WHO Collaborating Centers for Influenza.

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