How does the flu shot work?

The vaccine in the flu shot contains either a "dead" (inactive) or a weakened form of the expected invading virus (or viruses) which can not make an otherwise healthy person sick. Our bodies use these antigens (vaccine's virus particles) as a template to create the right antibodies, that will have the right pattern to match and know how to destroy those same viruses in the wild (normal strength), if they enter the body in the future to try to use our cells to replicate themselves. Our bodies remember each virus pattern that is in the vaccinations so that they can make those matching cells to protect us again from that same specific flu strain (or sometimes strains that are very similar).

Our bodies also have the ability to determine when a virus has invaded and, if there is a match to that strain of virus stored from a prior invasion, the immune systems quickly make up a lot of those attack cells, that worked in the past to destroy the invading virus particles. If it is a new invading virus, usually our bodies can make cells that can match just right to destroy the new invader in a week to ten days. (They can sometimes do this even more quickly for bacteria and other microbes).

It is during this time period, when the body is matching and making the attack cells to "kill" the invader, that we feel sick until our body finds the key pattern of cells that will match and destroy that specific virus. The body "remembers" the pattern each time it makes cells that work to kill germs, so any time that same germ comes along again, or even sometimes a similar one, the immune system wastes no time in getting rid of the invading organism, because it knows already what will work. That is why, once we have had a flu virus like H1N1/09 Swine Flu before, we won't get sick from it again. The body will already know what cells to make and will send them out to destroy the invader.

This is also how vaccines help our bodies build up immunity or resistance to viruses and flu. The immune system remembers the pattern of the antibodies it made when it was exposed to the virus particles in the flu shot and if that type of virus turns up in the environment later, the cells are made and attack the virus before it can make us sick.