Interfering activity of virulent and attenuated influenza virus strains.

The interfering activity of influenza virus variants A/Hong Kong/1/68 (H3N202), A/Victoria/35/72(H3N2-3), B/14/55 and B/USSR/69 differing in the level of their reactogenicity for adults and children was studied. An inverse relationship was established between reactogenicity of the strains and their interfering activity in the resistant chick embryo cell (CEC) cultures. Virulent strains did not interfere with vesicular stomatitis virus. Vaccine strains used for commercial live influenza vaccine safe for adults but reactogenic in children were intermediate and showed moderate interfering activity. The highest capacity for interference was demonstrated in cold-adapted thermosensitive variants non-pathogenic for both adults and children. The interfering activity of the attenuated strains increased progressively with increasing inocula.