The center-of-mass strains identified for the target seasons of 2001.0 to 2014.5 are listed in Table 1. The
average antigenic distances of these strains to the epidemic viruses sampled in their target seasons ranged
from 0.253 to 2.355 with the overall average of 0.887, which were always smaller than those for actual
vaccines (Table 1). The center-of-mass strain turned out to be the most effective vaccine strain for 7 out of
28 seasons examined (Table 1). In each pre-target season, the average antigenic distance for the center-ofmass
strain was greater than those for the other strains only in 13.0% of the cases on average (same: 13.9%;
smaller: 73.1%), suggesting that the center-of-mass strain was expected to be more effective than a strain
randomly chosen in the pre-target season (Fig. 1). Vaccine strains for the future epidemic seasons of 2015.0
and 2015.5 could also be identified as the center-of-mass strains, which were A/Guam/3712/2013 and
A/Virginia/10/2014, respectively (Table 1).