Studies on the P-Q phases of A/Asia/57 influenza viruses.

Some 300 strains of A/Asia/57 influenza virus were tested for haemagglutination-inhibition against normal human convalescent serum and showed wide variations; some, indeed, were not inhibited at all. The same strains, when tested against rooster antisera obtained by immunization with one of three strains (Adachi, K9 and Y5), again showed considerable variation in their degree of haemagglutination-inhibition. These two series of variations were fairly closely correlated.Eight strains considered representative of the range of inhibition variation were further tested against a number of human convalescent sera and against sera from persons inoculated with monovalent Adachi vaccine; the results indicated that the order of variation did not change.Neutralization tests to determine the immunizing potency of vaccines prepared from Adachi, K9 and Y5 strains showed that Adachi vaccine had far the greatest immunogenicity and that K9 was slightly more potent than Y5. Mouse adaptation of Y5 strain considerably enhanced its immunizing potency, but did not alter its reaction to human convalescent serum.