Naturally occurring temperature-sensitive influenza A viruses of the H1N1 and H3N2 subtypes.

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