Calicivirus can give your kitten a nasty upper respiratory infection. Vaccinating against it can stop the virus or at least make the infection not so severe. Some kittens do have reactions to the vaccine, but in most cases the reactions are not as bad as getting sick from the virus.
It's the quickest and the most serious reaction your kitten can have to the calicivirus vaccine, and it could be deadly. An allergic reaction can cause your kitty's throat to suddenly swell, making it hard for him to breathe. It could swell so much that your kitten can't get enough oxygen. Fortunately, if kitty does have an allergic reaction, it will probably happen before you leave the vet's office. If you aren't at the vet's, get there as soon as possible so he can get treatment.
It's not common, but a sarcoma, or cancerous growth can appear at the site where the calicivirus vaccine was injected. You will feel a small lump under the skin that doesn't go away. It's normal for a small bump to form right after the vaccine is given. That's a minor reaction and is nothing to worry about. If the bump is still there after a month, it's time to check with the vet.
Kittens who get sick with calicivirus often develop a limp. In the same way, your kitty may also develop a limp as a reaction to the vaccine. It is not serious and should go away within a day or two. If the limp does not go away, or your kitten seems to be in pain, have him examined by your veterinarian.
A lot of kittens will have some sort of reaction to the calicivirus vaccine, but usually these reactions are mild. A low-grade fever, tenderness at the injection site, sore joints and a poor appetite are typical symptoms. As long as your kitty is still drinking water and does not act depressed or lethargic, there's no need to worry about these reactions. If he will not drink, stops eating altogether and has no desire to run and play at least part of the time, it's a good idea to have him checked by your vet.