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The components of annual seasonal influenza vaccines are determined by best guesses based on viruses circulating earlier in other parts of the world. Although these guesses frequently are accurate, occasionally — as in this influenza season — they are not.
For the 2014–2015 influenza season, the recommended vaccines include two strains of influenza type A and one or two strains of influenza type B. But most of the A (H3N2) strains circulating this year are similar to A/Switzerland/9715293/2013, rather than to the strain in the current vaccine (A/Texas/50/2012). This mismatch is presumably attributable to antigenic drift, which occurs through mutation and is common among influenza viruses.
The early and widespread circulation of influenza viru…