Antibodies to the strain-specific and cross-reactive determinants of the haemagglutinin of influenza H3N2 viruses. Antiviral activities of the antibodies in biological systems.

Antibodies to the strain-specific (SS) and cross-reactive (CR) antigenic determinants of the haemagglutinin (HA) of influenza H3N2 viruses were prepared, and their antiviral properties in experimentally infected mice and in eggs were investigated. The SS antibodies were considerably more efficient than the CR antibodies in direct virus neuralization tests. The difference in the activities of the two types of antibody preparations was less pronounced in virus growth inhibition studies in the allantois-on-shell system and in passive protection studies in mice give antibody before challenge with virus. Nevertheless, even in these tests SS antibody was approximately four-fold more effective than was CR antibody in protection against fatal infection. Passive administration of CR or SS antibody preparations to mice challenged with sublethal virus doses, reduced the peak pulmonary titres of virus on day three post-infection compared with the control animals. The degree of reduction in titre exerted by the SS antibodies was greater than for the CR antibodies. In contrast, the rate of clearance of pulmonary virus 9-12 days after infection in the mice treated with SS antibody was slower than for the CR treated animals. Also, in SS antibody treated animals viruses isolated from mouse lungs showed evidence of minor antigenic variation away from the parent virus.