Live influenza A/Victoria/75 (H3N2) virus vaccines: reactogenicity, immunogenicity, and protection against wild-type virus challenge.

Four live influenza A/Victoria/75 (H3N2) recombinant virus vaccines were administered intranasally to a total of 50 volunteers who had little or no detectable serum neutralizing antibody. A recombinant with ts-1[E] having a 38 degrees C shut-off temperature caused febrile reactions or systemic reactions or both in 21% of the volunteers, but one with ts-1A2 having a 37 degrees C shut-off temperature caused no illness. Two recombinants prepared with cold-adapted A/Ann Arbor/6/60 caused 9% febrile reactions or systemic reactions or both. Virus shedding occurred in a minority of the 50 volunteers, but 90% developed a serum neutralizing antibody response. Wild-type A/Victoria/75 virus challenge of 34 of the vaccinated volunteers and 12 others who had had prior natural A/Victoria/75 virus infection revealed similar and significant protection when compared with the 96% infection and 68% febrile illness or systemic illness or both observed in 25 unvaccinated volunteers with little or no serum antibody. These results encourage continued efforts toward development of live influenza virus vaccines.