| Mutation | HI effect |
| L172Q | 0.39 |
| M251V | 0.37 |
HI data can be displayed as color on the tree or viewed via the tool tips that show when moving the mouse over a circle corresponding to a virus. To explore the HI titer data, select HI distance from focus in the color by menu and click on one of the available reference viruses indicated by grey squares. The tree will then be colored by log2 distance from this reference virus. The coloring either reflects the the direct measurements of HI titers provided by the WHO collaborating centers (notably the annual and interim reports by the NIMR in London), or models that are fit to these data. Whether the raw data, the tree model and the mutation model are used to color the tree can be chosen via the radio button on the left. If more than one measurement is available, we take the average over all available measurements. In the process of fitting the models, column (serum potency) and row (virus avidities) effects are estimated. These corrections can be subtracted from the raw measurements to remove noise. To see all measurements of a virus relative to the chosen reference virus, put the mouse over that virus and a info box (tooltip) will pop up with a table that lists all measurements (and the autologous titers for the sera to facilitate interpretation) and the model predictions.
The tree can also be colored by cumulative antigenic change -- similar to dimension 1 in antigenic cartography.
Use the date slider to select viruses sampled within the time interval indicated. The size of the interval can be changed by grabing the left end of the bar with the mouse, to move the interval, use the right end of the slider.
Use the drop down menu to color viruses by number of epitope mutations, non-epitope mutations or receptor binding mutations relative to root, or to color viruses by local branching index or geographic region.
Use the input box to specify positions to color viruses by genotype. Amino acid positions must be separated by a comma (e.g. 159,225). The default is HA1, to color by amino acid sequence in other regions use HA2:18 or SigPep:6. To color by nucleotide sequence, use nuc:527.
Mouse over a tip to show virus name, location and features.
Mouse over a branch to graph the frequency of the correponding clade trajectory below or click on a branch to zoom into its descendent clade. The tool tip will show amino acid mutations on this branch.
To restrict the displayed viruses to certain geographic regions, select the region in the drop down menu labeled region.
Epitope mutations are based on HA structure and exposed residues. Multiple recent mutations at epitope sites have been suggested to be predictive for strains dominating future seasons. Similarly, mutations outside of these epitopes -- termed non-epitope sites --- tend to be damaging and are suggested to be predictive of clade contraction.
Antigenic evolution has been shown to depend primarily on substitutions surrounding the receptor binding site of HA1. These seven positions (145, 155, 156, 158, 159, 189, 193 in HA1 numbering) are referred to here as receptor binding positions and changes at these positions could correspond to large changes in antigenic properties.
The local branching index is the exponentially weighted tree length surrounding a node, which is associated with rapid branching and expansion of clades. A more detailed explanation is available here. Retrospective analysis has shown that LBI correlates with clade growth.
Frequencies are estimated as maximum likelihood trajectories that penalize rapid changes in frequency and slope. The frequencies of large clades or abundant genotypes have sufficiently many observations to by robust, while frequencies of rare mutations can't be reliably estimated.