Properties of A/Victoria/3/75 recombinants: development of an attenuated strain RIT 4050.

An attenuated Influenza A strain, RIT 4050, has been selected among the inhibitor resistant variants of a range of H3N2 recombinants of A/PR/8/34 and A/Victoria/3/75. The criterion used for the selection of the vaccine strain was the homology rate of the viral RNA of the recombinant with the complementary RNA of A/PR/8/34 as determined by an RNA-RNA hybridization technique. Safety has been assessed by administering the vaccine by the nasal route to double seronegative volunteers. Incidence and nature of post-vaccinal symptoms was low and mild as in healthy seropositive volunteers. The excretion pattern of the strain was investigated. Volunteers shed virus for one day after vaccination and the titre of the virus was low. Eight reisolates of RIT 4050 were characterized and found similar to the original vaccine strain offered to the volunteers.