Temperature-sensitive mutants of influenza virus. XIII. Evaluation of influenza A/Hong Kong/68 and A/Udorn/72 ts and wild-type viruses in tracheal organ culture at permissive and restrictive temperatures.

For development of an in vitro assay of attenuation for virus vaccine candidates, tracheal ring organ cultures from young ferrets were infected with influenza A/Hong Kong/68 and A/Udorn/72 temperature-sensitive (ts) recombinant and wild-type viruses and observed for 21 days at permissive (33 C) and restrictive (37 C) temperatures for effects on ciliated epithelium and viral replication. Results were compared with the known effects of these viruses in humans. Viruses that damaged ferret cilia and grew to high titers at 37 C were insufficiently attenuated for humans, and viruses that failed to damage cilia and grew to low titers at 33 C were excessively attenuated for humans. This system did not completely differentiate the relative virulence of A/Hong Kong/68 ts recombinants for humans but was successful for the A/Udorn/72 ts recombinants. This organ culture system has adjunctive usefulness for detection of most insufficiently attenuated influenza A vaccine candidates and for identification of excessively attenuated candidates that might contain non-ts lesions.