A young couple brought their four-month-old Labrador in for his second puppy vaccine, and by the next morning they were calling in a panic: he'd been unusually quiet since the shot, slept through dinner, and barely wanted to go for his morning walk.When I asked them to describe what they were seeing, I recognized it immediately — a completely normal post-vaccination response in a puppy who'd just had his immune system appropriately stimulated. Read our guide to pet vaccinations: the complete for detailed information.
But their concern was entirely reasonable. No one had explained to them what "normal" looked like for the day or two after vaccination, and so they interpreted tiredness as illness.
Knowing what to expect after vaccination is just as important as knowing which vaccines to give.
Normal, Expected Reactions
These are the reactions I reassure owners about routinely:
Lethargy and reduced activity (24–48 hours)
The most common post-vaccination change. The immune system's response to vaccination is work, and that work comes with temporary tiredness. A dog or cat that seems quieter than usual, less interested in play, or more inclined to sleep is responding normally. It resolves within 24–48 hours without treatment.
Injection-site soreness or mild swelling
The area where the vaccine was administered — typically the scruff of the neck for cats, or the lateral neck or rear limb for dogs — may be tender or slightly swollen. Your pet may flinch if touched there, move away when approached, or be reluctant to turn their head. This is normal local inflammation. It should resolve within a few days.
Mild fever
A temporary low-grade fever (up to about 1°C above normal) is part of the normal immune response. Your pet may feel warm to the touch, seem less interested in food, or drink slightly more water. This passes on its own within a day or two.
Reduced appetite on the day of vaccination
Many dogs and cats eat less on vaccination day, particularly if the experience was stressful. Appetite typically returns to normal the following morning.
None of these reactions require treatment. A quiet rest day, normal feeding, and access to fresh water are the right response.
Reactions That Need a Vet Call
These reactions are less common but worth calling your veterinary practice about: Our pet vaccinations: the complete guide covers this in depth.
Vomiting or diarrhea occurring within hours of vaccination
A single episode of vomiting without other symptoms may simply be a stress response. Persistent vomiting, multiple episodes, or diarrhea appearing within the first four hours after vaccination can be an early sign of an allergic reaction.
Facial swelling that isn't severe
Mild puffiness around the muzzle, eyes, or face appearing a few hours after vaccination is worth a vet call even if your pet seems otherwise fine. It may resolve on its own, but it can also progress.
Injection-site lump persisting beyond 3–4 weeks (cats especially)
Small, firm lumps at injection sites are relatively common and usually represent normal inflammation that resolves within weeks. However, a lump that persists beyond three to four weeks, grows rather than shrinks, or becomes attached to underlying tissue in a cat warrants prompt veterinary evaluation. This is particularly important in cats because of injection-site sarcoma risk (see below).
Emergency: Signs of Anaphylaxis
Anaphylaxis is a severe, potentially fatal allergic reaction. It's uncommon — estimated at approximately 1-10 reactions per 10,000 vaccine doses in dogs — but it can develop rapidly, usually within minutes to two hours of vaccination.
Signs include:
- Facial swelling, particularly around the muzzle, eyes, and ears
- Hives — raised, itchy bumps visible as the coat parts
- Pale or white gums
- Vomiting or diarrhea
- Sudden extreme weakness or collapse
- Difficulty breathing
- Excessive drooling
Any combination of these signs after vaccination is a veterinary emergency. Read our guide to pet vaccinations: the complete for detailed information. Don't wait to see if it improves — call your vet or the nearest emergency clinic immediately and go.
Treatment involves epinephrine, antihistamines, and corticosteroids, and most animals who receive prompt treatment recover fully. The danger is in delayed response. See also core vs non-core vaccines for dogs.
Most veterinary clinics recommend waiting 15–30 minutes after vaccination before leaving the premises specifically to observe for acute anaphylaxis.
Injection-Site Sarcoma in Cats
Vaccine-associated sarcoma (VAS) is a rare but serious complication specific to cats. These are locally aggressive malignant tumors that develop at injection sites — historically most often at the interscapular space (between the shoulder blades), though modern vaccination protocols now use distal limb and lateral body wall sites specifically to make these tumors more surgically accessible.
The incidence is estimated at approximately 1 in 1,000 to 1 in 10,000 vaccine doses, with higher risk associated with adjuvanted core vs non-core vaccines for cats compared to non-adjuvanted or recombinant products. The interval between vaccination and tumor development ranges from months to years.
What to watch for: Any lump at a previous injection site in a cat should be assessed against the "3-2-1" rule used by many feline practitioners:
- See your vet if the lump is present 3 months after vaccination
- See your vet if the lump is larger than 2 cm
- See your vet if the lump is growing at any point after the first month
Treatment when detected is aggressive surgical excision; radiation therapy and chemotherapy are sometimes added. Early detection significantly improves outcomes.
The solution to VAS risk is not to avoid vaccination — panleukopenia and other preventable diseases are far more dangerous than the statistical risk of sarcoma. The solution is to use non-adjuvanted vaccines where available, use appropriate injection sites, and monitor for lumps.
Animals Who May Need Special Consideration
Previous serious reaction: If your pet had anaphylaxis or a significant allergic reaction to a vaccine, discuss future vaccination with your vet before the next appointment. Options include pre-treatment with antihistamines or corticosteroids, observation under clinic conditions, switching vaccine products, or in some cases deferring vaccination and using titer testing instead.
Animals on immunosuppressive medications: Corticosteroids and other immunosuppressive drugs can blunt the immune response to vaccination and, in the case of modified live vaccines, potentially allow the vaccine organism to cause disease. Timing vaccination to periods when immunosuppression is at its lowest dose, or switching to killed or recombinant vaccine products, is typically appropriate.
Senior animals: Older pets with significant underlying disease may not mount as robust an immune response to vaccines and may be more likely to experience fatigue post-vaccination. The question of whether the benefit of continued vaccination outweighs this in very elderly, frail animals is worth discussing individually with your vet.
FAQ
How long do vaccine side effects last in dogs and cats?
Mild effects — lethargy, injection-site soreness, reduced appetite — typically resolve within 24–48 hours. If your pet is still noticeably unwell after 48 hours, call your vet.
Can I give my pet pain relief or antihistamines after vaccination?
Do not give any human pain relief medications (paracetamol/acetaminophen, ibuprofen, aspirin) without specific veterinary guidance — many are toxic to pets. If your pet seems uncomfortable, contact your vet who can recommend appropriate products at the correct dose.
Is it normal for a cat to have a lump after vaccination?
A small, soft lump in the first two to four weeks is fairly common and usually resolves. A lump that persists beyond a month, grows, or feels firm and attached to tissue should be examined by a vet promptly.
Can my pet get the disease from the vaccine?
In healthy animals, this is extremely rare with modified live vaccines and impossible with killed or recombinant vaccines. Modified live vaccines can occasionally cause mild disease-like symptoms (sneezing after intranasal bordetella in dogs, for example), but serious vaccine-induced disease in healthy, normal-risk animals is uncommon and typically mild.
My dog vomited once after the vaccine — should I be worried?
A single episode of vomiting without other symptoms is often stress-related and doesn't necessarily indicate an allergic reaction. However, if vomiting continues or is accompanied by facial swelling, lethargy beyond normal post-vaccine tiredness, or any of the anaphylaxis signs listed above, contact your vet.