When you receive your second covid-19 vaccination, you might experience fever, fatigue, headache, chills, nausea and muscle pain. And in most cases, that’s actually a good thing.

These common symptoms typically indicate that the vaccine has triggered a response by your immune system. And that’s exactly what it’s supposed to do. The first vaccine teaches your body how to react to the virus. After the initial dose, your body begins building its initial immune response, including producing antibodies. But with the second shot, which is the second exposure to the virus, the “big guns” of your immune system react, and the response tends to be more robust.

So if you feel sick after your shot, don’t fret. It just means that the vaccine, and your body, are doing their jobs.