Evaluation in children of cold-adapted influenza B live attenuated intranasal vaccine prepared by reassortment between wild-type B/Ann Arbor/1/86 and cold-adapted B/Leningrad/14/55 viruses.

A reassortant cold-adapted (ca) influenza B experimental live attenuated intranasal vaccine was evaluated for safety and immunogenicity in children by means of a blind, placebo controlled study. The vaccine contained the haemagglutinin and neuraminidase genes, and the gene for its non-structural proteins from wild-type (wt) B/Ann Arbor/1/86 virus, the contemporary strain at the time of the study. Other genes were derived from ca B/Leningrad/14/55 virus. No increase in illness rates was seen in the children from ages 3-15 years given vaccine at maximum potency (a one in two dilution of infectious allantoic fluid, having a titre of 10(7.0) EID50) compared to children given placebo. About 60% of seronegative children, ages 3-7 years, exhibited a detectible antibody response following one dose of intranasal vaccine, with the seroresponse rate rising to greater than 70% after two doses of vaccine. Immunogenicity was lowest in seropositive children age 8-15 years, reaching a maximum of 36% after two doses. Results indicated that the vaccine was highly attenuated, and probably of adequate immunogenicity for kindergarten age children. The lower immunogenicity in older children suggests the vaccine might be overly attenuated for use in school-age children who are more likely to have a history of prior natural infection with influenza B virus. Further clinical and epidemiological studies of protection are needed to fully assess this.