Selecting vaccine strains for H3N2 human
influenza A virus
H3N2 human influenza A virus causes epidemics of influenza mainly in
the winter season in temperate regions. Since the antigenicity of this
virus evolves rapidly, several attempts have been made to predict the
major amino acid sequence of hemagglutinin 1 (HA1) in the target
season of vaccination. However, the usefulness of predicted sequence
was unclear because its relationship to the antigenicity was unknown.
Here the antigenic model for estimating the degree of antigenic
difference (antigenic distance) between amino acid sequences of HA1
was integrated into the process of selecting vaccine strains for H3N2
human influenza A virus. When the effectiveness of a potential vaccine
strain for a target season 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 season one year before the target season (pre-target season).
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 strain having the smallest average antigenic distance to the
others in the pre-target season. The procedure of selecting vaccine
strains for future epidemic seasons described in the present study was
implemented in the influenza virus forecasting system (INFLUCAST)
(http://www.nsc.nagoya-cu.ac.jp/~yossuzuk/influcast.html).