SUMMARYFrom January 1985 to May 1991, herald strains of influenza B virus were isolated in 1987 and 1989 in Japan. In both cases, influenza epidemics caused by the same type followed in the next winter season. The HA gene sequences of the influenza B viruses isolated in Japan from 1987–91, which covers two herald waves of influenza B viruses, were analysed and located on the phylogenetic tree for influenza B viruses after the B/Singapore/64 strain. Co-circulation of at least two evolutionary lineages of the HA genes existed for influenza B viruses in Japan during the period of this study. The herald viruses in one wave (1987) were genetically close to the winter isolates and were considered to be the parental viruses for the following influenza season, while in the other wave (1989) winter isolates belonged to another lineage on which one of the herald viruses was located, but they were genetically and antigenically different from the herald viruses.

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SUMMARYFrom January 1985 to March 1989, off-season viruses of H1N1 and H3N2 subtypes of influenza A viruses were isolated on five occasions in Japan. The HA gene sequences of the influenza A(H1N1) and A(H3N2) viruses isolated in Japan from 1985–9 were analysed and the phylogenetic tree for each subtype virus was constructed to determine any genetic relationship between viruses isolated in off-seasons and the epidemic viruses of the following influenza seasons. In one instance with H1N1 viruses in 1986 and in two instances with H3N2 viruses in 1985 and 1987, the spring isolates were genetically close to some of the winter isolates and were considered to be the parental viruses of the following influenza seasons. However, even in these cases, influenza viruses of the same subtype with different lineages co-circulated in Japan.

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 evolutionary trees of the hemagglutinin (HA) molecule based on amino acid sequences of the HAI polypeptide were constructed with old and new H1N1 viruses isolated 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 originate from the viruses of the previous H1N1 epidemic season but originated 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 viruses 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 history of old H1N1 viruses, in the 1999/2000 H1N1 epidemic season, the H1N1 viruses which were located on the same lineage as the previous epidemic viruses were also isolated sporadically in Japan.

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The epidemiology of influenza A in Japan was studied during 1979-91 and viruses isolated from reinfections during 1983-91 were analysed. Of 2963 influenza viruses isolated from reinfections during 1983-91 were analysed. Of 2963 influenza viruses isolated during this period, 922 and 1006 were influenza A(H1N1) and A(H3N2) viruses respectively; the others were influenza B viruses. Influenza A(H1N1) and A(H3N2) caused 5 and 6 epidemics respectively, most accompanied by antigenic drift. Seventeen reinfections with H1N1 and 17 with H3N2 were detected during our study. The primary and reinfection strains isolated from 7 H1N1 and 10 H3N2 cases were studied by haemagglutination-inhibition, and amino acid and nucleotide sequences of the HA1 region of the haemagglutinin. Most of the primary and reinfection strains were antigenically and genetically similar to the epidemic viruses circulating at that time. However, in 4 out of 10 cases of reinfection with influenza H3N2 virus, reinfection strains were genetically different from the epidemic viruses.

Two H1N2 recombinant viruses were isolated by a plaquing method from a throat swab of a patient who was simultaneously infected with HlNl and H3N2 influenza viruses during the Tokyo epidemic of 1981. This is the first direct evidence that recombination of influenza viruses occurred in the human body.

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