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 am hesitating between 4 options to answer the following question, which option should I choose?
Question: Why do doctors suggest that people get a flu vaccine each year?
Possibilities:
- 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.

Viruses can mutate from year to year.