The sore arms, fevers, and headaches are a result of innate immunity, the first of the immune system’s two main branches. Innate immunity is a blunt tool; it wants to fight anything foreign. When you get a COVID-19 vaccine, the cells in your arm take up mRNA that encodes a version of the coronavirus’s spike protein. The virus uses spike protein as a key to get into our cells, but unattached from the rest of the virus, the spike protein can’t infect anything. Still, the innate immune system recognizes the vaccine materials and the resulting spike protein as foreign.

This signal sets off a reaction that can feel a lot like getting sick. More immune cells get recruited to your arm, which may become inflamed and sore, activating even more immune cells that might cause whole-body symptoms such as fever and fatigue.

Some of those immune cells will belong to the second branch of the immune system, adaptive immunity. These are the targeted assassins of the immune system. They include B cells, which make antibodies that can bind to the spike protein and T cells that can recognize infected cells. Adaptive immunity is what will specifically protect you from COVID-19. And to get there, you first need the innate immune system to recognize the foreign protein and turn on the adaptive immune system. A reactogenic vaccine isn’t necessarily a more effective vaccine, but it is a sign that the first step is working.