A new recombination system for the production of live attenuated influenza-A vaccine strains is described. The attenuated parent is the Japanese strain A/Okuda/57 (H2N2) which has a known passage history and a good record of human safety. This virus has been recombined with two recent field isolates, A/England/42/72 and A/Finland/4/74, and in each case several recombinants were isolated and tested in volunteers. These recombinants differed in their degree of attenuation, but on each occasion it was possible to choose one that was suitably attenuated for a candidate vaccine strain. The genetic stability of such a recombinant produced with the A/England/42/72 strain was demonstrated by subjecting it to three successive human passages. A single intra-nasal dose of the A/Finland/4/74 recombinant vaccine gave a seroconversion rate of 75%.
This recombination system offers considerable advantages for the production of live influenza virus vaccines.