Viruses replicate more rapidly over time. Viruses can mutate from year to year. Vaccines are absorbed by the body after a year. Vaccines get stronger over time.
I gave my students this multiple choice question: Why do doctors suggest that people get a flu vaccine each year?

Only one answer is correct among these 4 choices:
- Viruses replicate more rapidly over time.
- Viruses can mutate from year to year.
- Vaccines are absorbed by the body after a year.
- Vaccines get stronger over time.

Could you tell me which one is correct?

Viruses can mutate from year to year.