Comparison of the haemagglutinin production of influenza A2 Hong Kong variant and recombinant strains.

Seven Hong Kong variant strains were compared for their capacity to produce haemagglutinin (HA). From the results of 9 serial egg passages made at 33 degrees C and 36 degrees C, the A2/Aichi/2/68 strain was found suitable as the vaccine production strain for the 1968-69 monovalent influenza vaccine manufactured in the United States of America.Results of initial experiments comparing the HA production by 2 Hong Kong variant recombinants and by the A2/Aichi/2/68 strain showed no essential difference among these strains. When these experiments were repeated starting with viruses that had undergone the same number of egg passages from isolation, the mean HA and chicken cell agglutination titres of the 3 recombinant strains used in these studies were only 1.5 times greater than those of the A2/Aichi/2/68 strain. By comparison, the HA producing capacity of A0/PR/8/34 was almost 6 times that of the A2/Aichi/2/68 strain.