Protective efficacy of RIT 4025, a live attenuated influenza vaccine strain, and evaluation of heterotypic immunity to influenza A viruses in ferrets.

Ferrets immunized with an H3N2 recombinant of A/PR/8/34 and A/Scotland/840/74 (RIT 4025 vaccine strain) were almost completely protected against a challenge with the homologous strain A/Scotland/840/74. The protection was lower but highly significant when the challenge was performed with the heterologous A/Victoria/3/75 wild strain. The protection afforded by the vaccine strain was measured by three indicators: absence of temperature rise, absence or reduction of virus shedding and absence or reduction of nasal protein increase when compared with uninoculated controls. Heterotypic immunity in this animal model was not significant when these three indicators were measured after a challenge inoculation performed 5 weeks after immunization.