Vaccines are among the most rigorously tested products in veterinary medicine. Serious reactions are rare. Most dogs experience nothing more than mild soreness at the injection site and temporary tiredness, and these resolve within 24–48 hours.
Normal post-vaccine reactions:
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Mild swelling or tenderness where the shot was given
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Low energy and reduced appetite for a day
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Mild fever
Reactions to call your vet about:
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Facial swelling, hives, or itching that begins within 30–60 minutes (signs of an allergic reaction)
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Vomiting or diarrhea after a vaccine appointment
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A firm lump at the injection site that persists more than 3 weeks
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Breathing difficulty (rare, go immediately)
If your dog has had a previous reaction to a vaccine, always tell your vet before any future vaccines. They can pre-medicate, use a different formulation, or split up vaccines across visits.
Vaccines cannot give your dog the disease they protect against. Modified live vaccines contain weakened virus that cannot cause full disease in a healthy immune system. The only exception is the intranasal Bordetella/parainfluenza vaccine, which can rarely cause mild, brief cold-like symptoms, this is expected and not a concern.