Antigenic Drift Requires Annual Vaccines

Steps:

• Influenza virus mutates its surface antigens (hemagglutinin and neuraminidase) through antigenic drift.
• These mutations alter the virus structure, making it different from previous strains.
• The immune system, primed by prior vaccines, fails to recognize the new antigens effectively.
• Thus, a new vaccine matching the current strain is needed for immunity.

Why D is correct:

• Antigenic drift is the gradual mutation of viral antigens, evading pre-existing antibodies as defined in virology, requiring updated vaccines yearly.

Why the others are wrong:

• A: Memory cells remain effective long-term; aging isn't the issue for influenza.
• B: Primary response produces sufficient antibodies; the problem is viral change, not quantity.
• C: Secondary response does produce antibodies, but against old antigens only.

Final answer: D