Hi! Fever is a part of body's defence. There are umpteen causes of fever. Also there are many strains of viruses. Vaccines are usually for the strains that are more serious. For a child the body is facing new and different challenges in terms of Micro organisms everyday  So the body will react. One vaccine cannot stop all and there are no Vaccines for all. Besides as us these microorganisms are also evolving and becoming tougher. A child can get fever cough cold etc for his body is learning to fight. Hope this helps.