The first dose of the COVID-19 vaccine is likely to bring on some soreness and mild flu-like symptoms. The second dose, however? It’s likely to kick a bit harder. Many people who have completed both doses note that the second one brings on more soreness and fatigue, feeling more like an actual illness.

However, this means it’s working as intended. Experts remind patients that these symptoms are typical not of a viral infection, but of the body’s response to a viral infection. What you’re feeling after a vaccine injection isn’t the effect of a virus, just the body’s rush to generate an immune response.

The heightened reaction after the second shot makes sense. After all, your body has now made considerable antibodies to handle the specific intruder the vaccine suggests to it. As such, the harder kick of the second dose just indicates that the vaccine is having its intended effect and is eliciting a strong, natural immune response.