The influenza virus, commonly referred to as the flu, is an rna virus that infects birds and mammals. upon infecting a host cell, the virus copies its rna genome using a viral polymerase. once the rna is copied, it is translated by the host cell into the viral proteins needed to package additional viral particles. there are three strains of influenza, but most humans in the united states are infected with strain a. this particular strain constantly changes, due to the high mutation rate of the rna copying machinery and the ability of the viral rna segments to undergo reassortments. although there are different influenza vaccines offered each year, it is estimated that the influenza virus still infects between 5% and 20% of the united states' population annually. which characteristic of the influenza virus best explains why there is a different influenza vaccine offered each year?