Question

why must the influenza vaccine be updated every year?
a. the influenza virus adapts and mutates every year, so new vaccines must be produced.
b. in some years, the influenza vaccine kills the virus, while in other years it does not.
c. in some years, the influenza vaccine is not as strong as in others.
d. the influenza vaccine does not necessarily stimulate the immune system to produce antigens.

Explanation:

The influenza virus is known to mutate and adapt over time. Vaccines are designed to target specific antigens of the virus. When the virus mutates, its antigens change. So, new vaccines need to be produced each year to match the current strain of the virus. Option B is incorrect because vaccines do not "kill" the virus in the sense described (they prime the immune system). Option C is not the fundamental reason - the issue is virus mutation, not vaccine strength variability in an arbitrary sense. Option D is wrong because vaccines do stimulate the immune system to produce antibodies (not antigens; antigens are on the virus, antibodies are produced by the body).

Answer:

A. The influenza virus adapts and mutates every year, so new vaccines must be produced.