摘要：During annual in. uenza epidemics, in. uenza B viruses frequently co-circulate with in. uenza A viruses and in some years, such as 2005, large outbreaks have occurred while in other years, the virus virtually disappears. Since 1987 there have been two lineages of in. uenza B viruses co-circulating in various countries and causing disease in humans. The proportions of these two lineages vary from year to year and country to country. For example, in 2005, the B/Victoria/2/87 lineage was predominant in New Zealand while in Australia the B/Yamagata/16/88 lineage was more common. Antigenic and genetic analysis has revealed gradual movement in the both lineages. Careful monitoring of the two virus line-ages is important, as they are antigenically distinct. This is an important consideration for in. uenza vaccine formulation decisions, as only one in. uenza B component is traditionally included in the annual trivalent in. uenza vaccine
