system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:  If the flu didn't evolve, would one vaccine be enough for one's entire life?
gpt
Yes.
Assuming you weren't looking for a simple yes/no answer, is your question "How do vaccines work?" If so:

People are born with a full immune system composed of cells, glands, organs, and fluids that are located throughout their bodies to fight invading bacteria and viruses. The immune system recognizes germs that enter the body as "foreign" invaders, or antigens, and produces protein substances called antibodies to fight them. A normal, healthy immune system has the ability to produce millions of these antibodies to defend against thousands of attacks every day, doing it so naturally that people are not even aware they are being attacked and defended so often. Many antibodies disappear once they have destroyed the invading antigens, but the cells involved in antibody production remain and become "memory cells." Memory cells remember the original antigen and then defend against it when the antigen attempts to re-infect a person, even after many decades. This protection is called immunity.

Vaccines contain the same antigens or parts of antigens that cause diseases, but the antigens in vaccines are either killed or greatly weakened. When they are injected into fatty tissue or muscle, vaccine antigens are not strong enough to produce the symptoms and signs of the disease but are strong enough for the immune system to produce antibodies against them. The memory cells that remain prevent re-infection when they encounter that disease in the future. Thus, through vaccination, children develop immunity without suffering from the actual diseases that vaccines prevent.