Mild reactions that can occur
Some pets are quieter than usual, have mild tenderness at the injection site, or eat slightly less for a day. A small local swelling can also occur. These signs should improve rather than progressively worsen.
Signs of an allergic emergency
Facial or muzzle swelling, hives, difficulty breathing, pale gums, collapse, severe weakness, or repeated vomiting and diarrhea can indicate an acute hypersensitivity reaction. Contact an emergency veterinarian immediately.
What about a lump at the injection site?
Small transient lumps may occur, but any mass that is enlarging, painful, persistent, or otherwise concerning should be examined. Cats have specific considerations for persistent injection-site masses, and your veterinarian may recommend follow-up based on size and duration.
Should a previous reaction stop future vaccination?
Not automatically. The veterinarian can review what happened, which vaccine was given, the pet’s disease exposure risk, and whether changes in scheduling, vaccine selection, or observation are appropriate. Decisions should balance infectious-disease risk with the individual reaction history.