A low fever for two days following your Tdap booster immunization is a good sign. Vaccines, like any medication, can cause side effects like fever. The fever shows that the body is responding to the vaccine. However, not getting fever doesn’t mean the vaccine hasn’t worked. Solution A low fever for two days following your Tdap booster immunization is a good sign. Vaccines, like any medication, can cause side effects like fever. The fever shows that the body is responding to the vaccine. However, not getting fever doesn’t mean the vaccine hasn’t worked..