virus. Similarly, A/H2N2/1957-derived virus disappeared after the appearance of A/H3N2/1968 pandemic virus containing six gene segments (PB2, PA, NP, NA, M and NS) from human A/H2N2/1957-derived virus. In 1977, A/H1N1/1918-derived virus reemerged, probably due to accidental release from a laboratory, as a low-grade A/H1N1/1977 pandemic virus that primarily affected young immunologically naïve people [17,18] who were born after the end of the H1 period (