Pandemic Vaccine Preparedness—Have We Left Something Behind?

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. The degree of relatedness between the strains contained in the seasonal vaccine and the drifted strains that are isolated the following year is assessed through cross haemagglutination inhibition (HI) tests. If the results of these tests suggest that cross HI levels are low, the vaccine is updated to include the most recent strains. We reasoned that if a semi-annual review of the antigenic characteristics of human H1 and H3 viruses is necessary