Most years, a new version of the seasonal flu vaccine is required because of:

• A. Antigenic drift.
• B. Antigenic shift.
• C. Hemagglutinin reshuffling.
• D. Neuraminidase reshuffling.

Answer: (A)

Verified step by step guidance

1. Understand the terms related to influenza virus variation: Antigenic drift and antigenic shift are two mechanisms by which the influenza virus changes its surface proteins, hemagglutinin (HA) and neuraminidase (NA).
2. Antigenic drift refers to small, gradual mutations in the genes encoding HA and NA proteins. These minor changes accumulate over time and can alter the virus enough to evade the immune system, necessitating updates to the vaccine.
3. Antigenic shift is a more abrupt, major change that occurs when two different influenza viruses infect the same cell and exchange genetic material, leading to a new subtype with a novel combination of HA and/or NA proteins. This can cause pandemics but is less frequent.
4. Hemagglutinin reshuffling and neuraminidase reshuffling are not standard terms; they likely refer to the genetic reassortment events involved in antigenic shift, but the correct scientific terms are antigenic drift and antigenic shift.
5. Since the seasonal flu vaccine is updated most years due to small, continuous changes in the virus, the correct mechanism responsible is antigenic drift.