Ferrets were found to react with a sharp febrile response to intranasal infection with influenza virus A2/Hong Kong/3/68. Virus was recovered from nasal washings taken 3 days after infection, and virus antibody was found in serum specimens taken 21 days after virus infection. Virus infection produced a pronounced rhinitis; the protein concentration in nasal washings was found to increase three to five-fold with peak levels occurring on day 7, post-infection. Concomitant with the increased protein levels, detectable levels of HI and neutralizing antibody were found in the nasal washings. However, nasal washings taken 13 days or more after influenza virus infection did not contain either increased levels of protein or detectable antibody. These ferrets were immune to re-infection with homologous virus inoculated 5 weeks after primary infection. Thus, ferrets showed no febrile response; virus was not recovered from nasal washings; serum antibody titres did not increase; no increase in protein levels was found in nasal washings; and HI antibody was not found in nasal washings.
Using these criteria to assess susceptibility or immunity to influenza virus infection, infection with attenuated influenza virus A2/Hong Kong/1/68 produced immunity to re-infection with virulent virus. Ferrets infected with influenza virus B/England/13/65 or immunized with killed A2/Hong Kong virus did not induce any immunity to infection with influenza virus A2/Hong Kong/3/68.