  Human seasonal influenza viruses evolve rapidly, enabling the virus
population to evade immunity and re-infect previously infected individuals.
Antigenic properties are largely determined by the surface glycoprotein
hemagglutinin (HA) and amino acid substitutions at exposed epitope sites in HA
mediate loss of recognition by antibodies. Here, we show that antigenic
differences measured through serological assay data are well described by a sum
of antigenic changes along the path connecting viruses in a phylogenetic tree.
This mapping onto the tree allows prediction of antigenicity from HA sequence
data alone. The mapping can further be used to make predictions about the
makeup of the future seasonal influenza virus population, and we compare
predictions between models with serological and sequence data. To make timely
model output readily available, we developed a web browser based application
that visualizes antigenic data on a continuously updated phylogeny.
