Comparison of the evolution of recent and late phase of old influenza A (H1N1) viruses

A comparison of the evolutionary tree of new influenza A (H1N1) viruses to that of old H1N1 viruses which disappeared in 1957 was performed, The evolu tionary trees of the hemagglutinin (HA) molecule based on amino acid sequen ces of the HA1 polypeptide were constructed with old and new H1N1 viruses i solated from 1947 to 1957 and 1986 to 2000, respectively. The evolutionary history of recent H1N1 viruses was similar to that of old H1N1 viruses just before the disappearance in two respects. Firstly, both viruses did not or iginate from the viruses of the previous H1N1 epidemic season but originate d from the viruses branched off at the same point on the mainstream stem as the viruses of two H1N1 epidemic seasons earlier Secondly, recent H1N1 vir uses mainly circulating in Japan have a deletion at amino acid residue 134, located close to residue 131, which was deleted in old H1N1 viruses at the time of the disappearance. However, different from the evolutionary histor y of old H1N1 viruses, in the 1999/2000 H1N1 epidemic season, the H1N1 viru ses which were located on the same lineage as the previous epidemic viruses were also isolated sporadically in Japan.