What causes fever four days after receiving the flu vaccine?

Generally, fever refers to an elevated body temperature. Developing a fever four days after receiving the influenza vaccine may be caused by vaccine side effects, exposure to cold, or a common cold. The specific causes are analyzed as follows:

1. Vaccine side effects

The influenza vaccine works by introducing antigens to stimulate the body's immune system, triggering a corresponding antigen-antibody response and producing protective antibodies. Some individuals may experience adverse reactions after vaccination, such as fever. In most cases, these symptoms resolve spontaneously over time and do not require special treatment.

2. Exposure to cold

After vaccination, the body's immunity might be relatively low. If the body is exposed to cold conditions at this time, the immune system may become stimulated, affecting the body's temperature regulation center and leading to an increase in body temperature.

3. Acute gastroenteritis

If the body is invaded by bacteria, viruses, or other pathogens after vaccination, acute gastroenteritis may develop, which can also cause fever.

In addition to the above reasons, fever could also result from upper respiratory tract infections. It is recommended to seek timely medical evaluation to confirm the diagnosis and receive appropriate treatment.