XK and infection: The large variations in both the HA and NA sequences during the transitional period between Mos/10/99 and Fuj/411/02 (Figure S3 and S4) may have reduced the protective efficacy of any pre-existing influenza antibodies (from prior infection or immunization by Mos/10/99-like viruses) before the inclusion of the Fuj/411/02 strain in the World Health Organization (WHO) recommendations for the seasonal influenza vaccine in 2004.