In vitro markers for measuring residual virulence for men of live attenuated influenza viruses.

Human and equine influenza strains attenuated by consecutive passages in the presence of normal horse serum (NHS) on allantois-on-shell system (AOS) and administered to human volunteers or horses as possible live virus vaccine candidates were tested in organ cultures of ferret and hamster trachea. Temperature sensitivity, virus replication, interferon induction and neuraminidase activity were also investigated. Observation of the ciliary activity of each ring was made daily. Influenza strains which were incompletely attenuated following two to five passages on AOS + NHS system and caused severe to mild symptoms of influenza in volunteers or horses also caused a 50% inhibition of the activity of the ciliated epithelium earlier than did completely attenuated strains following ten passages on the same system. The A/Hong Kong/68 strain attenuated by Beare and Bynoe was used as a reference strain in every test. This technique appears to be valuable for screening live influenza vaccine candidates and may prevent risking severe illness in volunteers. No significant results could be ascertained with other markers investigated.