evidence: Mild-to-moderate side effects, like a low-grade fever or muscle aches, are normal and not a cause for alarm: they are signs that the body’s immune system is responding to the vaccine, specifically the antigen (a substance that triggers an immune response), and is gearing up to fight the virus. These side effects usually go away on their own after a few days. Common and mild or moderate side effects are a good thing: they show us that the vaccine is working. Experiencing no side effects doesn’t mean the vaccine is ineffective. It means everybody responds differently.