Final answer:
Annual influenza vaccines are necessary due to rapid antigenic variation in influenza viruses, requiring new formulations to match prevalent strains each year.
Explanation:
Antigenic variation in influenza viruses leads to the continual emergence of new strains that the immune system cannot recognize, requiring annual new influenza vaccines to protect against the most prevalent strains for that year. This rapid antigen mutation in influenza makes it difficult for existing immunity to provide adequate protection.
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