Swine recovered from infection with either swine influenza or swine influenza virus alone are usually not only immune but refractory to human influenza infection. Swine recovered from infection with a mixture of human influenza virus and H. influenzae suis are usually immune to swine influenza while those recovered from infection with human influenza virus alone are usually not immune to swine influenza. The possible mechanisms involved in the cross-immunity between the influenza viruses are discussed.

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