The first one, some of your cells get told by the mRNA/adenovirus vector (depending on if you had Moderna/Pfizer or J&J/Oxford) to make some 'rona spikes. Your immune system amped up a little bit and your cells feel bad for a bit because of having to spew out spike. Your immune system, which is a little amped up, sees all this and starts to formulate things to kill the spikes (antibodies) and starts to send out other immune cells that clamp down on the cells that are making the spikes.
The second shot? Your cells get the call from the mRNA/adenovirus vector and start spewing out spikes. Your immune system recently saw this so it _really_ starts spewing out more antibodies to get rid of the spikes and more of the other cells to get rid of the spike producers. It it is really ready for the fight and wants to make sure you're ready for the fight in the future too.
The negative side effects are your immune system causing parts of your body to do things it doesn't usually do (like raising your body temperature to help kill off cells that are making these "rogue" spikes).
As every medical expert has been saying for months now, the vaccine doesn't effect your DNA.
The immune response is what causes the side effects, and the immune response is stronger with the second dose (this follows intuitively from immunity being more robust after 2 doses).
The mRNA molecules in the vaccine just contain the instructions for the spike protein. https://berthub.eu/articles/posts/reverse-engineering-source... has a good breakdown of what specificly it makes.
To use an analogy to a computer - DNA is like the hard drive (long term storage but slow), RNA is like RAM (short term, degrades easily, but easy to transport and gets info where it needs to go), ribosomes are like the CPU (executes the code in the RNA, outputs a protein). Normally, DNA gets copied into RNA, which instructs the ribosome what protein to make. The vaccine is like inserting a fake message. The ribosomes see RNA and follow the instructions, but the RNA came from the vaccine instead of the cell's DNA. This is one of the reasons why the vaccine is so safe, because RNA just naturally breaks down very quickly in the body.
Booster generally causes more side effects, because the body has seen the spike protein before (from the first vaccine) so it recognizes it faster and mobilizes a more robust response. In a certain sense, its already done a practise run, so the body kicks things into high gear right away.
[IANAD]
After I read it, I noticed that it was the first time anyone had ever tried to explain _exactly_ how these new vaccines worked, like down to the molecular level.
Most explanations I had seen were usually extremely high level (literally just "the RNA gets read by your cells, which then produce the spike protein, then the RNA is destroyed") or used analogies (as you have done here).
I don't consider myself a conspiracy theorist, but I admit I had been harbouring some concerns/fears over these new vaccines. This link is the only thing I have read that actually allayed those fears.
I think there is a lesson here.
I have a feeling that everyone fears stuff they don't understand, at least a little bit.
I know that the high level explanation is technically correct, but here's the problem: Many conspiracy theories are actually _extremely_ detailed. I have personally seen covid conspiracies that appear to be backed up by scientific papers.
This is what the truth is competing against. A basic, high level "ELI5" explanation is never going to convince a conspiracy theorist who is looking at a stack of science papers.
So I think this link is very important, more people need to read and share it.
BTW: different people have different immune systems, and their body reacts differently. I got my first dose in the morning. By afternoon I just felt tired and wanted sleep (I felt like I didn't sleep the previous night). I went to sleep and next day felt normal. For 2nd dose the same thing happened, except I felt like that one extra day. My vaccine was Pfizer.
Everyone says the second dose is worse although.
The nurse was a talker, and distracted me from watching what she was doing. I didn't see the shot go in, and I felt absolutely nothing. I only realized it was done when she said I did not bleed at all and didn't need a bandage.
Every other shot I've ever received I felt, but every other one I was also watching when it happened. Now I'm wondering if feeling them was imaginary, something my brain made up because I saw the jab and expected to feel it.
Not to detract from people who did have side-effects, merely pointing out that it's possible to have none.