Abstract
The immunogenicity of several cold-adapted (ca) viruses was compared in CSL mice with that of wild-type parental viruses with similar surface antigens, according to the vaccinating dose required to clear a challenge consisting of 10(4.5) 50% tissue culture infective doses of the wild-type virus. All ca viruses were less immunogenic than their wild-type parental strains by a factor of 10(1.3) to 10(3.4), probably due to the restricted capacity of ca viruses to replicate in the respiratory tracts of mice. However, their immunogenicity was considerably enhanced when two quite small doses were administered 3 weeks apart. The immunogenicity of ca viruses when administered in two doses and wild-type viruses when administered as a single dose varied according to their surface antigens. It was highest for viruses with the H2N2 A/Ann Arbor/6/60 and H3N2 A/Queensland/6/72 surface antigens and lowest for those with H1N1 A/HK/123/77 surface antigens. When two doses consisting of 10(5.0) 50% tissue culture infective doses of A/Ann Arbor/6/60-ca were administered at an interval of 3 weeks, solid immunity was induced against the wild-type A/Ann Arbor/6/60 parental virus, two heterologous H3N2 strains, and an H1N1 strain.