I'm back from that client with the information I need for their tax 
Thanks Moonie and Summer 
I've had the flu jab before. I may get a bit of muscle ache in the shoulder where I had the jab overnight but that doesn't last long. Flu jabs are made each year based on what is thought to be the more likely flu strains over the coming winter. There are quite a number of strains of flu virus and the viruses do mutate to a new variant. So flu jabs can't guarantee that someone won't get flu. However, if one gets infected with a different strain the seriousness of the infection is generally lessened as is the duration of the illness.
As the flu strains vary and mutate that's why there's the need to get a flu jab each year rather than just a one off.