The good thing is that vaccine related sarcoma, though agressive, is extremely rare, especially in the UK. There seem to be a few 'clusters' in the US where it is more common (though still rare) and apparently some US vets inject in the leg rather than the scruff of the neck on the basis that if the cat does develop sarcoma, a limb can be amputated rather than lose the cat. I did a lot of reading up on it when The Munchkin developed a lump after an injection. In his case it was just a reaction, went down after a week or two and has never happened since.