Final answer:
A new flu shot is required each year because of the high mutation rate of influenza viruses, leading to antigenic drift and new virus strains that the immune system has not encountered. Scientists and pharmaceutical companies work together to create and mass-produce vaccines targeting the predicted prevalent strains for each flu season.
Step-by-step explanation:
The need for a new flu shot each year is primarily due to the high mutation rate of influenza viruses. Such viruses possess surface molecules that undergo continuous mutations, enabling them to evade the immunity developed by previous vaccinations. This phenomenon is known as antigenic drift, where small changes in the viruses' genes cause the creation of new virus strains that the human immune system has not yet encountered and, therefore, has no immunity against.
Scientists globally collaborate to identify the flu strains that are expected to be most prevalent in the upcoming flu season. They then develop a vaccine that targets these specific strains. Pharmaceutical companies quickly mass-produce this vaccine to provide protection for the public before the flu season commences.
Compared to other viruses with a slow mutation rate, such as those causing measles, mumps, and rubella, the influenza virus requires annual vaccine updates to keep up with its rapid antigenic variation, ensuring effective protection against the newly evolved strains.