Seventeen of twenty-six influenza A virus isolates of the H1N1 antigenic subtype and two of eleven H3N2 virus isolates from the 1977–78 season exhibited a phenotype, were restricted in plaquing in MDCK cells at 38.5 °C compared to 34 °C and appeared to be naturally occurring mutants. The cut-off temperature for two such H1N1 virus isolates was established as 38 °C. The viruses were as thermostable as non- isolates and no complementation was detected between the twelve viruses tested. Cloning studies with an H1N1 virus isolate with minimal properties indicated the presence of a mixed population of and particles. Analysis of seven recombinants of A/HK/117/77 (N1N1) virus indicated that the lesion(s) was not located in the NA or HA proteins.


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