Exploration of the emergence of the Victoria lineage of influenza B virus.

The Victoria lineage represented by B/Victoria/2/87 is one of the two major distinctive haemagglutinin (HA) lineages of influenza B virus, and its recent re-emergence has aroused great concerns. However, it remains unknown when, where, and how this HA lineage emerged in the world. In this study, the HA1 domain of the HA gene of fourteen influenza B viruses isolated in China in 1972-1984 was sequenced. The sequences were phylogenetically analyzed with the HA1 sequences of 41 other important influenza B isolates. The results unveiled some earlier footprints of the Victoria lineage in China, and the epidemic history of the Victoria lineage could be traced back from the year 1985 to 1975. Moreover, phylogenetic analysis, the history of China, and the epidemiology of influenza B virus indicated that the Victoria lineage possibly emerged in China in the 1970s through gradual evolution from a minor lineage.