mic virus, A/H1N1/1918-derived virus, disappeared from human circulation after the appearance of the A/H2N2/1957 pandemic virus containing five gene segments (PB2, PA, NP, M and NS) from human A/H1N1/1918-derived virus. Similarly, A/H2N2/1957-derived virus disappeared after the appearance of A/H3N2/1968 pandemic virus containing six gene s