  Influenza is a contagious respiratory illness that causes significant human
morbidity and mortality, affecting 5-15% of the population in a typical
epidemic season. Human influenza epidemics are caused by types A and B, with
roughly 25% of human cases due to influenza B. Influenza B is a single-stranded
RNA virus with a high mutation rate, and both prior immune history and
vaccination put significant pressure on the virus to evolve. Due to the high
rate of viral evolution, the influenza B vaccine component of the annual
influenza vaccine is updated, roughly every other year in recent years. To
predict when an update to the vaccine is needed, an estimate of expected
vaccine effectiveness against a range of viral strains is required. We here
introduce a method to measure antigenic distance between the influenza B
vaccine and circulating viral strains. The measure correlates well with
effectiveness of the influenza B component of the annual vaccine in humans
between 1979 and 2014. We discuss how this measure of antigenic distance may be
used in the context of annual influenza vaccine design and prediction of
vaccine effectiveness.
