An attenuated influenza virus vaccine: protection against homologous and heterologous strains of virus.

An effective influenza vaccine should be capable of providing protection against both the homologous virus strain and heterologous strains representing antigenic "drift". Two attenuated vaccines were evaluated, an A/Hong Kong/8/68 (H3N2) and an A/England/42/72 (H3N2) strain. Volunteers were immunized intranasally with either placebo or vaccine in a "double-blind" fashion in two doses, 2 weeks apart. Eighty-four subjects were challenged 30-100 days after the second dose with either the homologous or a heterologous strain. The heterologous strain for the A/Hong Kong/8/68 (H3N2) vaccinees was the A/England/42/72 (H3N2) virulent strain. The heterologous strain for the A/England/42/72 (H3N2) vaccinees was a virulent A/Dunedin/73 (H3N2) strain. Both vaccines led to good protection against both homologous and heterologous challenges. The protection rate against illness for the A/Hong Kong/8/68 (H3N2) vaccinees was 73% and 100% following homologous and heterologous challenges, respectively. The protection rate for the A/England/42/72 H3N2) vaccinees was 100% following both homologous and heterologous challenges. The protection rates against infection (as judged by antibody responses, irrespective of signs and symptoms) were also good. For the A/Hong Kong/8/68 (H3N2) vaccinees the rates were 73% (homologous) and 86% (heterologous). For the A/England/42/72 vaccinees, the rates were 72% and 60% respectively. Thus, immunity induced by these attenuated influenza vaccines extends to provide protection against related but non-identical influenza viruses.