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

An effective influenza virus vaccine should be capable of inducing protection against both the homologous strain of virus and the heterologous strains representing antigenic drift. For evaluation of the protection provided by an attenuated influenza virus vaccine, volunteers were immunized intranasally with either placebo or influenza A/England/42/72 (H3N2) virus vaccine in a double-blind fashion in two doses administered two weeks apart. Forty-seven subjects were challenged 30-100 days after the second dose with either the homologous or a virulent heterologous strain of virus [A/Dunedin/73 (H3N2)]. The vaccine resulted in good clinical protection against challenge with both the homologous and heterologous strains of virus. The rates of protection against infection (as judged by antibody responses, irrespective of signs and symptoms) were also good (72% and 60% against the homologous and heterologous strains, respectively). Thus, immunity induced by this attenuated influenza virus vaccine extends to provide protection against a related but nonidentical influenza virus.