What is a common side effect associated with live vaccines?

Live vaccines, which contain a weakened form of the virus or bacteria that causes disease, often trigger the immune system to respond as though there is an actual infection present. A common response to this immune activation is the development of a mild fever. This is typically a normal and temporary reaction, indicating that the body is building immunity.

Fever occurs as the body's immune system mobilizes to combat the weakened pathogen, which can result in an increase in body temperature. The elevated temperature is usually a sign that the vaccine is working to stimulate the immune response, preparing the body to recognize and fight off the disease in the future.

In contrast, severe allergic reactions are generally more associated with other types of vaccines or medications, prolonged bleeding is not a typical side effect of vaccines, and impacted immunity suggests a state where the immune response may be hindered, which does not align with the typical reactions to live vaccines.