Loading
A vaccine does not fight anything for you. It shows your immune system a picture, so it can practise before the real thing arrives.
Meeting a new germ for the first time is slow. It takes days to work out what it is and build the right response.
Days is often enough time to become very ill.
Meeting a germ it has seen before. Then the response is fast, and mostly finished before you notice anything.
A vaccine arranges the second kind of meeting without the first one having happened.
That is the entire idea, and everything else is how.
What does an injected flu vaccine contain?
A flu shot cannot give you flu. An inactive virus cannot transmit disease. There is no version of it that could.
That is not reassurance, it is what inactive means.
Sore arm, tiredness, sometimes a mild fever for a day.
That is not illness. That is the rehearsal happening.
Most of what you feel with any infection is your own immune system working, not the germ.
So a small dose of that feeling after a vaccine makes sense. It is the same machinery, without the germ.
Sort these.
Tap an item, then a bucket
After the rehearsal your body keeps cells that remember. If the real thing turns up, the response starts immediately instead of from scratch.
Two reasons, and they are different. Protection fades over time. And the flu viruses circulating change, so last year's picture is out of date.
Some are given once or twice and last decades, because those germs change slowly.
The yearly schedule is about flu, not about vaccines in general.
Not that you will never catch it. A rehearsed response can still be beaten by a big enough exposure.
What it changes is how fast and how hard your side arrives.
It is a picture, not a fight. Nothing in the shot is alive. Feeling briefly rough is the practice, not the illness. And flu is annual because both you and the virus keep moving.