If a child develops a fever after receiving the diphtheria vaccine, it is mostly caused by an adverse reaction to the vaccination. The diphtheria vaccine is a combination vaccine to prevent pertussis, diphtheria and tetanus in children. The vaccination of children will cause the body to produce an immune response, and protective antibodies will be produced in the body to effectively prevent the occurrence of the corresponding diseases in children. However, the process of producing protective antibodies can cause an overwhelming immune response that can lead to fever in children. Most of the fevers in this case are not very high, rarely exceeding 39°C under the armpit, and most of them only need to drink warm water and rest properly. If the fever does not subside for more than 48 hours, a hospital visit is required.