[Differentiating markers of virulent and attenuated (cold-adapted) strains of influenza A virus].

A high sensitivity of influenza virus to replication at a high temperature and a reduction of the temperature optimum of the neuraminidase activity are important genetic markers correlating with the degree of attenuation for the man. Cold-adapted variants of influenza virus produced in the course of passages at a low (25-28 degrees C) temperature are thermo-sensitive. The passage of the viruses at the optimal temperature (32 degrees C) does not change the range of thermal sensitivity even in variants, harmless for adult persons. The interfering activity of influenza viruses with different levels of human virulence was studied. A considerable portion of the examined cold-adapted strains differs from the original virulent strains in the capacity to interfere with the challenge virus in chick embryo fibroblast cell culture. However some thermosensitive variants as well as original strains do not react in interference tests. This indicates the lack of complete correlation between the interfering activity, thermal sensitivity and human virulence of influenza virus.