Every year brings a new version of the vaccine for seasonal flu, because the virus can mutate and evade antibodies already present in the body. It might seem as though scores of mutations have contributed to that escape artistry. According to a new report, however, mutations to just seven amino acids are the ones that truly mattered over a 35-year period (Science 2013, DOI: 10.1126/science.1244730). All seven of these amino acids are in hemagglutinin, a glycoprotein on the surface of the flu virus. Six of the seven align next to one another on the protein’s receptor binding site. Derek J. Smith of the University of Cambridge, Ron A. M. Fouchier of Erasmus Medical Center in the Netherlands, and colleagues made this discovery in the H3N2 flu subtype, which contributes to seasonal outbreaks. In strains that emerged between 1968 and 2003, five of the seven key amino acids were involved in the strain’s evolution more than once. “These findings have potentially far-reaching consequences for understanding the evolutionary mechanisms that govern influenza viruses,” the team writes.