Hello and welcome to Ask A doctor Service. I reviewed your query and here is my answer. It looks like your child has fever due to vaccination in which case there is no need to worry about much as this may happen after vaccination. But it could be due to some sort of infection and cbc is first line and mandatory test for that. It will tell you about anemic status of the child. Besides this, the wbc count will tell you whether there is an ongoing infection or not. If there is increased neutrophil count, then it is most likely bacteria, but if there is increased or decreased lymphocytes, then it is more likely virus because virus can give you both increased or decreased lymphocytes. Hope i answered your question. Feel free to ask if you need further assistance. Thanks.