system
Given the following post, respond as if you're commenting on Reddit:
human
Covid vaccine 2nd dose?

Is the 2nd dose of the (Pfizer or Moderna) vaccine a different formulation than the 1st? Why do people get more side effects from the 2nd dose?
gpt
Both doses are the exact same formula. The reason you respond differently to the second dose than to the first dose is because your immune system is adaptive.

When you get the first dose, your immune system has never seen the COVID-19 spike protein before. 

Your body mounts an innate immune response, but does not have any adaptive response yet. 

The innate immune system is that part of the immune system that can’t differentiate one kind of pathogen from another, but just generally attacks anything that seems “foreign”. This is always your first line of defense against against any disease. 

The adaptive immune system is that part of your immune they learns from past experiences, and it includes those precious antibodies that specifically attack and neutralize specific pathogens that you have encountered at least once before. 

That adaptive response takes a few weeks after infection or vaccination for your immune system to build up. That process of building up an adaptive response is initiated by the first dose of the vaccine.

When you get the second dose, the immune system has already been primed by its previous encounter with the COVID-19 spike protein, has built up some adaptive immunity, and is now spoiling for a fight. 

So your body may react much more strongly to the second dose, as both the innate and adaptive responses kick in at once and much more robustly. And that stronger response may manifest as more side-effects.