Final answer:
The influenza virus mutates rapidly, making it difficult to provide complete protection against all strains. New vaccine formulations are developed each year to protect against prevalent strains, but new strains may emerge in subsequent years.
Step-by-step explanation:
Viruses, like the influenza virus, mutate rapidly, which makes it difficult for our immune system to recognize and provide 100% protection against all strains. Each year, the influenza virus undergoes antigenic variation, resulting in the emergence of new strains that our immune system may not recognize. Consequently, new vaccine formulations are developed annually to provide protection against the most prevalent strains for that year. However, new or different strains may circulate the following year, requiring a new vaccine.