A fever after vaccination can be alarming, especially when your dog suddenly feels hot, tired, or not quite like themselves.
While many post-vaccine reactions are mild, fever is one sign that should never be ignored.
We explain the reasons why fever after vaccination in dogs happens and what you can do at home to stabilize them, and why prompt veterinary attention is important.
Dog Fever After Vaccine
A dog developing a fever after a vaccine is usually the result of the immune system reacting strongly to the vaccine, but it can also signal inflammation, infection, or a more serious adverse response.
Vaccines work by stimulating the immune system, and in some dogs this activation becomes excessive, leading to elevated body temperature, weakness, and appetite loss.
Fever may also occur if bacteria were introduced at the injection site, if the dog is having an allergic response, or if the vaccine triggered an underlying health condition. Small dogs, older dogs, and immune-sensitive dogs tend to be more vulnerable to fever reactions.
A dog fever after vaccine happens because the body mistakes vaccine components as real threats and launches a full inflammatory response, which raises temperature and disrupts normal organ function.
Dog Fever After Vaccine: Common Causes
Immune System Overreaction
Vaccines are designed to activate the immune system, but in some dogs that response becomes too strong. Instead of a controlled reaction, the immune system releases large amounts of inflammatory chemicals that raise the body temperature.
This can make your dog feel hot, lethargic, and weak, and may be accompanied by shivering or loss of appetite. Dogs experiencing immune-driven fever often seem uncomfortable and withdrawn.
This is serious because sustained inflammation can strain the heart, kidneys, and liver if it continues unchecked.
Related: Dog diarrhea after vaccines (Is it normal?)
Injection Site Inflammation
Sometimes the tissue where the vaccine was injected becomes inflamed, triggering localized swelling and pain. That inflammation can spill into the bloodstream and cause a systemic fever.
Dogs may flinch when touched near the injection site, limp, or resist movement. You might notice warmth or a lump under the skin.
When inflammation spreads, it can lead to a full-body fever, which increases metabolic stress and dehydration risk.
Bacterial Infection at Injection Site
If bacteria entered the skin during vaccination, an infection can develop at the injection site. The immune system responds by producing fever to fight the invading organisms.
Signs may include redness, heat, swelling, pus, and worsening lethargy. The fever may keep climbing rather than improving.
Infections can spread into the bloodstream, which makes this a medical emergency if not treated quickly.
Allergic Reaction
Some dogs react strongly to vaccine ingredients such as preservatives or adjuvants. This reaction can trigger widespread inflammation and fever.
Along with fever, dogs may show facial swelling, hives, itching, vomiting, or breathing changes.
These reactions can progress rapidly into life-threatening shock if not treated promptly.
Autoimmune Flare Triggered by Vaccination
In dogs with underlying autoimmune tendencies, vaccines can trigger an immune flare that attacks their own tissues. Fever is often the first sign.
These dogs may become extremely tired, refuse food, or show joint stiffness and weakness.
This is serious because autoimmune inflammation can damage organs and require long-term management.
Underlying Illness Unmasked by Vaccination
Some dogs already have mild infections or chronic disease before being vaccinated. The immune stimulation from the vaccine pushes their body over the edge, causing fever.
You may notice symptoms that seem unrelated to the vaccine, such as coughing, diarrhea, or urinary issues.
This type of fever usually does not resolve on its own and needs veterinary evaluation.
Read more: Dog Vomiting After Vaccine (What it means and what to do)
What to Do If Your Dog Has A Fever After A Vaccine
Keep your dog in a calm, cool, and quiet environment so their body can focus on recovery. Avoid exercise or excitement for at least 48 hours after vaccination.
Make sure fresh water is always available, as fever increases fluid loss and dehydration makes symptoms worse.
You can gently check your dog’s temperature if you have a pet thermometer. A temperature over 102.5°F indicates fever and should be monitored closely.
Offer soft, appealing foods if appetite is low, but do not force eating. Rest is more important in the first 24 hours.
Never give human fever medications, as many are toxic to dogs and can cause kidney or liver failure.
When to Call or Visit Your Vet
Seek veterinary care if:
- Your dog’s fever lasts more than 24 hours after vaccination, it should be evaluated.
- Your dog is extremely lethargic, collapsing, or unable to stand, this is an emergency.
- The fever is combined with vomiting, diarrhea, facial swelling, or trouble breathing, immediate veterinary care is needed.
- The injection site becomes increasingly swollen, painful, or oozes fluid, infection may be present.
Small dogs, puppies, and senior dogs should be seen sooner because they decline faster when fever is involved.
Read more: Dog side effects after vaccines (Possible reactions explained)
Key Takeaway
A dog fever after vaccine is not something to brush off as “normal.” While mild immune reactions can cause brief temperature rises, persistent or severe fever often signals inflammation, infection, or an adverse vaccine reaction.
Monitoring your dog closely, supporting them at home, and knowing when to seek veterinary care can make all the difference in keeping them safe and comfortable after vaccination.