Pandemic vaccine preparedness--have we left something behind?

Since 1997, most scientific studies have focused on avian H5, and to a minor extent avian H7 and H9 subtype viruses, because these viruses have caused repeated zoonotic human infections and have spread widely in poultry over the past 10 years or more. There are significant antigenic differences within subtypes that change over time as these viruses evolve, and this requires a semi-annual review and frequent update of vaccine strain candidates for human seasonal influenza vaccines.