In the retrospective analysis, vaccine strains could be identified by the fitness model only for the target
seasons of 2011.0 to 2014.5, because of a failure in optimizing the model parameters due to relatively small
numbers of data points available for computation (Table 1). In addition, for seven out of eight seasons examined,
the average antigenic distance for the vaccine strain identified by the fitness model was greater than that
for the center-of-mass strain obtained above (Fig. 1). The vaccine strain for the future epidemic season of
2015.0 identified by the fitness model (A/Guam/3712/2013) was the same as the center-of-mass strain,
supporting the reliability. For season 2015.5, however, a different strain (A/Washington/3810/2014) was
identified by the fitness model, which was likely to be less reliable than the center-of-mass strain as suggested
from the results of the retrospective analysis (Table 1). Similar results were observed when either the crossimmunity
or the thermodynamic stability was allowed to be included in the fitness model (data not shown).