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Vaccine Reactions

Vaccine Side Effects 

Vaccinations save lives.  Modern vaccines are extremely effective and safe.  However, it is common for dogs and cats to experience mild side effects following vaccination, similar to those that humans experience. Rarely does a pet need veterinary medical attention for these normal, minor side effects. 

With vaccination commonly recommended annually, most pet owners are accustomed to taking their pets to the vet for "yearly shots." It seems such a commonplace part of routine pet care that many people do not think about what is occurring within their pet's body. In fact, immunization represents stimulation of the immune system, an inherently inflammatory process.  

Before immunization, tell your veterinarian if your pet has experienced any previous vaccine reactions, regardless of how insignificant they may seem. If you have any concerns that your pet may have a serious vaccine reaction, wait at your veterinary clinic for 30 to 60 minutes after vaccination. 

What are some common side effects of vaccination in companion animals? 

Some of the most common mild side effects seen after immunization include: 

  • Redness, mild swelling, and tenderness at the vaccination site.  Sometimes puppies will yelp when they are picked up or are sensitive to touch when they are petted.   

  • Decreased activity levels (fatigue).  Cats will especially lay around more than usual.  Puppies will not be so chipper post vaccinations. 

  • Loss of appetite.  Dogs and cats may skip a meal or not finish an entire meal.  This is usually very transient. 

  • Low-grade fever.  You may not even notice that your furry friend has a fever, but loss of appetite and fatigue are signs of fever.  Generally transient and resolves within 24 hours. 

  • Nasal discharge, sneezing, coughing, or other respiratory symptoms, which may occur up to 2 to 4 days after an intranasal vaccine (vaccination with drops or sprays via the nostrils) is administered.  Commonly our Bordetella Bronchiseptica vaccine is intranasal in administration.  


Over the counter Benadryl and Low dose baby aspirin can also help counteract or minimize the above side effects.  Our veterinarians can advise a dosage based on your patient’s body weight.    

If any of these minor side effects lasts more than 24 hours or if your pet appears extremely uncomfortable, please don’t hesitate to call our office or an emergency facility if after hours. 

Is a lump at the injection site normal? 

Vaccinations stimulate the immune system and upon injection into the patient, an inflammatory reaction occurs.  This reaction can rarely translate into a firm knot or bump at the injection site.  These growths are typically noted a few weeks following vaccination and generally resolve on their own. Monitor the size of the site and make sure the nodule is not painful or hot. If the area does not resolve in 2 – 3 weeks, schedule a medical progress exam for the veterinarian to check the area. 

Are there other possible side effects of vaccination that I should watch for in my pet? 

Other less common but more serious side effects can occur within minutes to hours after vaccination. These reactions are medical emergencies, and you should seek veterinary care immediately if any of the following occur: 

  • Collapse or fainting 

  • Difficulty breathing 

  • Persistent and severe coughing 

  • Small, red, raised, itchy bumps over the body (hives) 

  • Swollen or puffy eyes, muzzle, or face 

  • Persistent vomiting or diarrhea  


Take home message about vaccines 

The majority of the tens of millions of pets are vaccinated without any problems each year. Closely monitor your pet and report any problems to your veterinarian as soon as possible. Vaccines save countless lives and prevent devastating infectious diseases from threatening our pet loved ones. If you have any questions or concerns, please discuss them with your veterinarian. 

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