Abstract
Background: Analysis of the hemagglutinin (HA) and neuraminidase (NA) genes of influenza A and B viruses revealed worldwide circulation of multiple genetic lineages within the same type/subtype. Genetic reassortment between distinct genetic lineages was examined in this study. Methods: Nucleotide sequencing, polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) were used. Phylogenetic studies were used to analyze sequence data. Results: Influenza A (H3N2) . The NA genes of H3N2 viruses were found to have evolved as three distinct lineages represented by A/Sydney/5/97, A/Panama/2007/99, and A/Moscow/10/99, respectively. Similar genetic groups were also found among the HA genes of these viruses. PCR/RFLP data of strains isolated between June 1999 and June 2000 showed that 122 of 141 viruses examined (87%) had their NA genes closely related to that of A/Moscow/10/99 virus; 14 of 141 viruses (10%) had A/Panama/2007/99-like NA genes, and only 5 strains (3%) had A/Sydney/5/97-like NA genes. Viruses bearing A/Panama/2007/99-like HA, and either A/Moscow/10/99-like, or A/Sydney/5/97-like NA were detected, indicating that genetic reassortments of the NA genes among different lineages of H3N2 viruses had occurred. Influenza A (H1N1) . Analyses showed that the NA and HA genes of the current strains each evolved as two genetically distinct lineages, represented by A/Bayern/7/95 and A/Beijing/262/95, respectively. The NA genes of the majority of circulating H1N1 viruses were closely related to that of the current vaccine strain A/New Caledonia/20/99 virus (A/Beijing/262/95 lineage). Influenza B . The NA genes of influenza B viruses, similar to their HA genes, were found to have diverged into two genetically and antigenically distinguishable lineages represented by B/Yamagata/16/88 (recently B/Yamanashi/166/98) and B/Victoria/2/87 (recently by B/Shandong/7/97) strains. Reassortants bearing the B/Yamagata-like HA and B/Victoria-like NA were isolated from several countries. Conclusion: Monitoring both HA and NA genes/antigens is important for appropriate vaccine strain selection.
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