Important: VAERS reports alone cannot determine if a vaccine caused an adverse event. Reports may contain incomplete, inaccurate, or unverified information. Correlation does not equal causation.
Fever (pyrexia) is the most commonly reported symptom in VAERS, with 276,779 reports. In most cases, post-vaccination fever is a normal sign your immune system is working.
Fever after vaccination is your immune system doing exactly what it's supposed to do. When the immune system encounters the vaccine antigens, it mounts an inflammatory response that can raise body temperature. This is the same mechanism that causes fever during natural infections — just without the actual disease.
Not everyone gets a fever after vaccination. It depends on factors including age, the specific vaccine, which dose (second doses often cause more symptoms), and individual immune system variation.
Based on VAERS reporting data, vaccines most commonly associated with fever reports:
In young children (6 months to 5 years), fever from any cause can occasionally trigger febrile seizures. While frightening, febrile seizures are generally harmless and don't cause lasting damage. They occur in about 1 in 3,000 children after MMR vaccination — similar to the rate from any fever of the same temperature.