Both the hemagglutinin and neuraminidase proteins of the influenza virus are able to tolerate point mutations in their amino acid sequence without necessarily affecting the ability of the virus to replicate. Such a mutation (or coincident mutations) at one of the sites recognized by the host antibody response may result in the host antibody, induced by vaccination or a previous infection, being unable to bind effectively to the “new” virus strain thereby allowing an infection to result. As the human influenza strains are continually evolving via these point mutations, the virus is able to escape from the limited antibody repertoire of the human immune response and cause epidemics. For this reason, manufacturers must change the composition of the influenza vaccine on an annual basis in order to ensure an accurate match is achieved with the circulating viral strains; considerable efforts are therefore made to monitor epidemiological patterns across the world as a means of predicting the strains likely to be prevalent during the next seasonal epidemic.