“Recent Human Influenza A (H1N1) Viruses Are Closely Related Genetically to Strains Isolated in 1950”, 1978-07-27 ():
Comparison of the oligonucleotide maps of the RNAs of current human influenza (H1N1) virus isolates shows these strains to be much more closely related to viruses isolated in 1950 than to strains which circulated before or after that period.
…In contrast, comparison of the oligonucleotide maps of H1N1 viruses isolated in 1947, 1950 and 1956, revealed extensive genetic differences. These differences in the oligonucleotide maps of the RNAs of viruses isolated within the period of prevalence of one subtype could be due either to the concurrent presence of different variants each of which is genetically conserved, or to multiple mutational events occurring over the entire period. Although the first hypothesis cannot be excluded, for the following reasons we regard the latter explanation to be more likely. First, there is no evidence to show simultaneous prevalence of several markedly different genetic variants belonging to the same subtype. Second, the observation of small differences in the oligonucleotide maps of viruses isolated within a few months of one another suggests that detectable mutations are occurring during epidemic spread over a short period of time.
If we accept the premise that influenza A viruses are subject to repeated mutational events it is extremely difficult to explain why the oligonucleotide maps of strains isolated in 1950 and those of the recent Russian viruses are so strikingly similar. For the reasons stated above it seems unlikely that a 1950 virus survived by normal sequential transmission in the human population without evidence of much more extensive genetic drift. It is also not plausible to speculate that chance background mutations accidentally produced a strain so similar to those isolated in 1956.