My husband has sores / blisters in his mouth and into his throat as well as flu like symptoms of aching body and joints. Could this be from the Yellow fever injection he received on Wednesday? Should he go to hospital rather?

Mouth sore. Hi
Your husband needs to be examined
It sounds as if he has oral herpes simplex or coxackie viruses
Both are viral infections which are spread by coughing ,sneezing etc
Both are easily treated
It is very unlikely due to the vaccine that he received .