The major reason for the preparation of different influenza vaccines each year is that

While it's true that some viruses can have short lifespans outside a host, this is not the primary reason for yearly vaccine updates. Vaccines are designed to provide immunity against specific viral strains, and the primary issue is the rapid mutation of the influenza virus, not its lifespan.

Antibodies can evolve in response to infection, but this is not the reason for changing flu vaccines annually. The main issue is that the influenza virus itself changes, leading to new strains that may evade existing immunity. Thus, the modification of antibodies does not require an annual vaccine change.

While animal viruses can infect humans and contribute to new influenza strains, the primary reason for the annual vaccine update is the mutation of existing human influenza viruses. This choice oversimplifies the complexity of influenza transmission and vaccine formulation.