Figure 4. Influenza B circulation by lineage: US and European data for 2001 to 2011. Data were obtained as reported for Figure 1. US data are represented in panel A (A) and European data are represented in panel B (B). The influenza B lineage (Victoria or Yamagata) recommended for inclusion in the trivalent vaccine is shown on the x-axis for each season. EU data regarding the proportion of B viruses by lineage were not available for the 2001–2002 and 2002–2003 influenza seasons.