The behaviour in vitro of attenuated recombinant influenza viruses

Influenza strains produced by recombination and tested as possible live vaccine candidates were studied in organ cultures of trachea. Two strains which proved to be too virulent in human volunteers regularly caused damage to the ciliated epithelium and viruses grew to high titre. Two strains which proved to be attenuated for volunteers did not cause appreciable damage, although they replicated to low titre in the epithelium. Similar results were obtained with influenza A virus attenuated by passage in the presence of horse sera. The method may be of value for detecting virulent live influenza vaccine candidates without risking severe illness in volunteers.