Why does an influenza vaccine require annual updates?

• A. Antigenic drift changes surface proteins targeted by vaccines.
• B. The virus mutates in predictable cycles unrelated to surface proteins.
• C. Annual updates are required to adjust for changes in manufacturing methods.
• D. Influenza vaccines aim to cover all subtypes in one shot.

Answer: (A)

Influenza vaccines are updated yearly because the virus changes its surface proteins through antigenic drift. The immune response from previous vaccines targets those surface proteins, mainly hemagglutinin, so small mutations can let the virus slip past antibodies. To keep protection as high as possible, scientists review global flu activity and reformulate the vaccine to match the strains predicted to be most common in the upcoming season. It’s not about changes in manufacturing methods, and vaccines don’t try to cover all subtypes in one shot—the goal is to align protection with the strains most likely to circulate.