Live influenza vaccine: screening of attenuated virus strains by a 50% ciliary activity inhibition test in organ cultures of ferret trachea.

This study of three live attenuated inhibitor-resistant influenza vaccines showed that these preparations are usually antigenic and that they caused no significant reactions when characterized by an index of attenuation equal to or slightly better than 1.0 arbitrarily attributed to the 'reference' attenuated A/Hong Kong/68 strain of Beare and Bynoe. This index, measured in vitro on ferret tracheal rings, is expressed as the ratio of the time required for ciliary activity inhibition of 50% of the rings by the tested candidate vaccine strain and the 'reference' attenuated strain. Induction of heterologous antibodies was also observed. Oral administration of underattenuated perparations did not cause the severe reactions which were observed when the same vaccine was administered intranasally.