evidence: Vaccine Safety: Fever and Vaccines [...] Vaccines prepare the immune system to protect against viruses or bacteria that could make people sick. The way this happens is that they introduce components of the germs that are known to activate the immune response. However, vaccines will not cause a significant enough immune response that the person suffers untoward events, such as can occur during natural infections. With this said, in some cases the immune response is strong enough to cause detectable symptoms, like a mild fever. [...] Additional resources - Treating a Fever: What to Consider (video)