Association between Haemagglutination inhibiting antibodies and protection against clade 6B viruses in 2013 and 2015

In 2013 and 2015, antibody titers to A/California/07/09 associated with an infection risk reduction amongst exposed household contacts. This is consistent with a detectable vaccine effectiveness reported in a number of studies. Genetic changes in clade 6B viruses might have led to a reduced immunity in some whereas others might have been less affected. The use of human serologic data is important in virus characterization and if performed in a timely manner, could assist in vaccine strain selection.