Figure 4. Matching between circulating and vaccine strains of influenza B, Hong Kong, China, 1996–2012. Each circulating virus was assigned on the basis of full-length hemagglutinin amino acid distances and phylogenetic tree topology to the closest World Health Organization–recommended influenza B vaccine strain for Northern Hemisphere (A) and Southern Hemisphere (B) vaccines. Closely matched viruses are labeled with the same color. The circulating strains with no closest vaccine strain identified as defined in the Methods are indicated by dotted boxes. Horizontal color bars indicate the protection period of the corresponding vaccine strain. Asterisks indicate vaccine strains without closely matched circulating viruses. The protection period of Northern Hemisphere (A) vaccines was defined as from November through the following October and of Southern Hemisphere (B) vaccines from May through the following April. All the alternative vaccine strains were analyzed, and those with different results are shown.