"Acute" and "convalescent" samples of serum from 40 patients from a localized outbreak of influenza were tested for mouse-protective antibodies against 7 different strains of influenza virus, which included 2 laboratory strains representative of "group A" and of "group B," and 5 strains from the investigated outbreak. The latter 5 strains although not identical were related to each other and to the PR8 strain. The chief point shown by the data was the considerable degree of antigenic difference among the 6 "group A" strains, evidenced by the marked differences in the protective capacities of the serums when tested against the various strains.

A number of the "acute" serums showed high protective capacity against some strains but relatively little protective capacity against other strains. In the 5 instances in which it was possible to test the "acute" serums against strictly homologous strains of virus, no protective capacity was demonstrable. In 7 of 36 cases, the antibody responses evoked by influenza infections were not detectable by tests with some strains, but were detectable by tests against other related strains.

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