Cold recombinant influenza B/Texas/1/84 vaccine virus (CRB 87): attenuation, immunogenicity, and efficacy against...

Healthy susceptible young adults were inoculated intranasally with increasing doses of wild-type influenza B/Texas/1/84, or the cold-adapted vaccine possessing the genes specifying the hemagglutinin and neuraminidase of the wild-type parent and the six internal genes of cold adapted B/Ann Arbor/1/66 (CRB 87). Most volunteers inoculated with 10(6.6)-10(7.6) TCID50 of CRB 87 were infected, but a high frequency of serum antibody responses was seen only at the highest dose (17/29; 59%). The dose of CRB 87 necessary to infect 50% of all human volunteers (1 HID50) was approximately 10(5.4) TCID50. All volunteers given 10(3.9)-10(7.1) TCID50 of the wild-type virus were infected (i.e., 1 HID50 was less than 10(3.9) TCID50). The frequency of mild febrile reactions, mean peak titer of virus in respiratory secretions, and duration of virus shedding were significantly greater in volunteers given 10(7.1) TCID50 of wild-type virus than in those given 10(7.6) TCID50 of CRB 87. Thirteen volunteers were rechallenged with a second 10(7.6) TCID50 dose of CRB 87 3-4 months after vaccination. The frequencies of mild upper respiratory symptoms and signs, virus shedding, and infection were significantly reduced in prior vaccinees compared with volunteers inoculated with a similar dose for the first time. These data suggest that CRB 87 is attenuated, immunogenic, and can confer protection against homotypic virus challenge in this susceptible population.