Recent human influenza A (H1N1) viruses are closely related genetically to strains isolated in 1950

@article{Nakajima1978RecentHI,
  title={Recent human influenza A (H1N1) viruses are closely related genetically to strains isolated in 1950},
  author={Katsuhisa Nakajima and Ulrich Desselberger and Peter Palese},
  journal={Nature},
  year={1978},
  volume={274},
  pages={334-339}
}
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. 
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