What is a common side effect of vaccination?

Fever is a common side effect of vaccination because it is a natural response of the immune system to the introduction of an antigen. Vaccines work by stimulating the immune system to recognize and fight off pathogens. As the immune system activates, it can cause a temporary increase in body temperature, leading to a mild fever. This response indicates that the body is building protection against the disease targeted by the vaccine. While other options may have their own associations or implications, they are not commonly linked to vaccination side effects in the way fever is. Understanding that fever can be a normal reaction following vaccination helps to distinguish between expected outcomes and those that may require further medical evaluation.