I know here in the U.S. they decide what flu jab or how we call it in the U.S. , flu shot  is made up of. I know our’s is for the respitory influenza. They choose the string of influenza that is more likely to be popular for that season, create it and then distribute it. My daughter is 6 and a half, as I mentioned below, has SWCAH and is just recently OVER supressed. She has received the flu shot ever since she was of age to receive it which I cannot remember it right now but I think it was like 2 years old.  I have also heard here in the U.S. that if you are first time recipient of the flu shot and I THINK it is if you are under a certain age, you need to receive the shot in two seperate doses like a couple of weeks apart. That is how I can remember my daughter receiving it for the first time. I am not sure how the UK distributes these shots or as you call them jabs (I don’t know which sounds more painful)  Both my pediatrician and endocrinologist suggested she get this shot every year. Every year no matter what the medical staff says, she gets a bit sick from it. She just had her flu shot a couple of weeks ago and sure enough she got sick. They say it is coincidence. I say it is not. They say more respitory flu’s kill more than any other illness in a year AND hospitalize hundreds. So the flu shot is a good idea. My daughter is sick yet again today. She is running the same fever she did a few weeks ago. Since starting kindgarten she has missed so many days of school already because of these cold viruses. I am debating whether or not to lock her in a plastic bubble is has been so horrible. Just a note here for your son. You may need to get his throat cultured to see if he could have strep throat. Many a times if a child gets sick with a fever, sore throat and vomitting, it is a pretty good sign of strepachochis.(spelling is way off, but in the U.S. we are used to calling it plain old strep throat.) Good luck! Get the jab!