When mice are immunized by one intraperitoneal inoculation with active or inactive influenza virus (strain PR8, W.S., and Melbourne) the quantity required for protection against heterologous strains is about 10 times the homologous minimal immunizing dose. Three injections increase the immunity to all strains, but the ratio between the homologous and heterologous minimal immunizing dose is not altered.

Swine influenza virus given intraperitoneally fails to immunize against human strains unless the quantity injected is 1,000 times the minimal amount required for homologous immunity.

Intranasal immunization of mice with 1/100 M.L.D. of attenuated ferret passage strains PR8 and Philadelphia, or the tissue culture strain of swine influenza, gives a solid resistance to infection with heterologous strains. When smaller amounts of virus are given intranasally, strain specificity becomes more apparent, and with minimal doses the immunity may be effective only against the homologous and closely related strains.

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