In the present study, the effectiveness of a potential vaccine strain for a target season of H3N2 human
influenza A virus was evaluated retrospectively using the average antigenic distance between the strain and
the epidemic viruses sampled in the target season. The most effective vaccine strain was identified mostly in
the pre-target season and the effectiveness of actual vaccines appeared to be lower than that of the strains randomly
chosen in the pre-target season on average. It was recommended to replace the vaccine strain for every
target season with the center-of-mass strain in the pre-target season. Recently, complicated algorithms based on
the phylogenetic analysis have been devised to identify the strains that were likely to be more effective than
actual vaccines in the pre-target seasons (Luksza and Lassig, 2014; Steinbruck et al., 2014). However, the strains
isolated in the pre-target seasons were expected to be more effective than actual vaccines as indicated above,
and thus it may be more meaningful to identify the most effective vaccine strains in the pre-target seasons.
The procedure described in the present study provides a simple way directed toward this purpose.