Vaccine reaction in dogs and cats

Recognise the signs

• NORMAL (does not require veterinary contact): mild lethargy or quietness for 24–48 hours; reduced appetite; small, firm lump at the injection site (should resolve within 2–4 weeks); mild local soreness
• MODERATE (call your vet): facial swelling, particularly around the muzzle, eyes, or ears; hives (raised bumps under fur); vomiting or diarrhoea within 1–4 hours post-vaccination; persistent lethargy beyond 48 hours
• SEVERE — ANAPHYLAXIS (emergency vet within minutes): collapse or extreme weakness; pale, white, or grey gums; breathing difficulty or wheezing; extreme distress; cold extremities; vomiting with collapse; typically occurs within 15–30 minutes of vaccination
• DELAYED CONCERN — injection site sarcoma (cats): any lump at a vaccination site that persists beyond 4 weeks, is larger than 2 cm, or continues to grow = urgent veterinary assessment

First aid steps

1. If your pet is having a routine vaccination, wait at the practice for at least 15–30 minutes post-injection — this is when anaphylaxis is most likely to occur and the team can respond immediately.
2. For mild normal reactions: provide a quiet, comfortable place to rest. Offer water and gentle encouragement to eat. Monitor for the next 24–48 hours.
3. For moderate reactions (facial swelling, hives, vomiting): telephone your vet practice immediately and describe the signs. Do not wait and see — antihistamines and/or corticosteroids may be needed promptly.
4. For anaphylaxis (collapse, breathing difficulty, white gums): go to the nearest emergency vet immediately — do not wait for your own practice to open. Call ahead if possible so they can prepare adrenaline (epinephrine).
5. For a lump at the injection site in a cat: monitor the size and duration. Any lump present after 4 weeks, over 2 cm, or growing = telephone your vet for an urgent assessment appointment.
6. Do not give any human antihistamines without speaking to a vet first — some (e.g. those containing xylitol or decongestants) are toxic to pets.

While transporting to the vet

• For anaphylaxis: carry the animal rather than letting them walk. Keep airway clear. Keep warm. Call the vet from the road. Drive directly to the nearest emergency vet — do not take time to call your regular vet first if it means delay.

When it can usually wait

• Mild lethargy and a small soft lump at the injection site are normal and resolve without treatment. Very mild, transient vomiting or reduced appetite in an otherwise alert and responsive animal may be monitored — but telephone your vet for advice if you are unsure.

Common causes

• Hypersensitivity (allergic) reaction to vaccine components — adjuvants, proteins, or stabilisers
• Anaphylaxis — a severe, rapid immune response (rare but life-threatening)
• Normal immune activation — causing mild, transient side effects
• Injection site sarcoma in cats — a rare but serious complication associated with certain vaccines (especially in the interscapular region)

What the vet will need to know

• Which vaccine was given and when
• Exact time post-vaccination that symptoms began
• Specific symptoms observed
• Current gum colour
• Whether the pet is conscious and responsive
• Any known previous vaccine reactions
• Current medications

Aftercare

• For anaphylaxis survivors: future vaccinations should be discussed carefully with your vet. Pre-treatment with antihistamines or corticosteroids before future vaccines may be recommended. Some pets may require titre testing to check immune status rather than routine booster vaccination. For injection site sarcoma: surgical excision with wide margins is required — this is an aggressive tumour and early intervention significantly improves prognosis.

Prevention

• Always stay at the practice for at least 15–30 minutes post-vaccination
• Inform your vet of any previous reactions before vaccination
• Keep your pet's vaccination records accurate so boosters are given at appropriate intervals — over-vaccination increases risk
• For cats: request vaccination in the lower limb rather than between the shoulder blades — if a sarcoma develops in the lower limb, it is more surgically accessible
• Keep a record of which vaccine brand was used in case of reaction — for future reference

Breed-specific notes

• Some small breeds — particularly Dachshunds, Chihuahuas, and Yorkshire Terriers — appear to have higher rates of vaccine reactions. Cats are specifically at risk of injection site sarcoma (feline injection-site sarcoma, FISS) — a rare but aggressive form of cancer. Modern feline vaccines are formulated to reduce adjuvant levels and lower this risk, but any persistent post-vaccination lump must be taken seriously.

Frequently asked questions

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