Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066


Allergic Reactions


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 actually occurring within their pet's body. In fact, immunization represents stimulation of the immune system, an inherently inflammatory process.


After vaccination, it is usual for pets to return home and re-engage in normal activity; however, since vaccination is inflammatory by nature, sometimes there are symptoms for a couple of days:

  • Pain at the vaccine site
  • Mild Fever
  • Lethargy
  • Reluctance to play or exercise
  • Reduced appetite
  • Sneezing after a nasal vaccine

These symptoms are considered normal for the vaccination process. Treatment is generally unnecessary, though sometimes an appropriate anti-inflammatory medication is needed for comfort. The patient can be expected to be back to normal in a couple of days. Again, if these reactions are seen, there is no reason to alter vaccination recommendations or plans for the patient's future as these symptoms are inherent to the vaccination process and are considered normal.

Pepper’s facial swelling is actually from an insect bite, but would look identical to the facial swelling of a vaccination reaction.

Pepper’s facial swelling is actually from an insect bite, but would look identical to the facial swelling of a vaccination reaction.
(original graphic by



Occasionally, a reaction is extreme and potentially dangerous. As vaccine technology has advanced, these reactions are more and more rare but it is important to know when to take action. The following symptoms are more serious and one's veterinarian should be notified at once, especially if they occur within hours of vaccine administration:

  • Vomiting or Diarrhea
  • Facial swelling
  • Hives
  • Collapse
  • Difficulty breathing

These symptoms can be part of an "anaphylactic" reactions which is more extreme and potentially life-threatening in its most severe form. Any symptoms from this list should be taken seriously and steps should be taken to alter future vaccination plans to avoid more severe problems. These types of vaccination reactions tend to get worse with subsequent exposure to the offending protein. The reaction can be directed against the infectious organism, vaccine stabilizers, preservatives, or residue from the vaccine organism's laboratory tissue culture.


In October 2005, a study by Moore et al was published in the Journal of the American Veterinary Medical Association where over 1 million canine medical records in over 350 separate animal hospitals were reviewed. In this study, approximately one in 250 dogs had some sort of vaccination reaction which translated to 13 reactions for every 10,000 doses of vaccine given.

The group at greatest risk for reaction were small breed young adult (age 1-3 years) neutered male dogs. As a dog's size increased, the risk of vaccination reaction dropped. Not surprisingly, the more vaccines were given at one time, the higher the risk of reaction. Some vaccine doses include vaccine against multiple organisms. These are called "multivalent" vaccines and include the basic Distemper-parvo vaccine for dogs and some Bordetella vaccines. Since giving more vaccines at once is associated with increased risk of reaction one might expect that the use of multivalent vaccines would also increase the risk of reaction, but surprisingly this was not found to be the case. Most reactions occur the same day as the vaccine is given.



If your pet is having a reaction more severe than just some general malaise or soreness, you should let your veterinarian know right away. If it is after hours, it is prudent to consult the local emergency clinic. Anti-inflammatory injections can be used to halt the inflammatory cascade before it gets dangerously out of hand. Be sure you know who to call in case of problems after your pet is vaccinated.

Vomiting may be a sign of an on-coming serious reaction,
or could represent something as mild as car sickness.
If vomiting is observed, let your veterinarian know right away.



After a pet has had a vaccine reaction, the question arises of what to do the next time vaccinations are due. There are several steps that can be employed as prevention.


Obviously, if you know your pet has had vaccination reactions in the past, be sure to inform the veterinary personnel of this so that proper premedication can be given. Do not wait until after vaccines have been given and THEN inform the staff that there is a history of reaction. It is also extremely helpful to know what the reaction consisted of. (A little lethargy, local soreness or mild fever is considered normal for the vaccination process and does not constitute a significant reaction.) If you know what vaccines preceded the reaction, the veterinary personnel will also need to know that.


Medications such and antihistamines or corticosteroids can be used to "head off" allergic reactions before they happen and with the use of these medications a reacting pet can be vaccinated normally. To be safe, it is frequently recommended that the pet be observed for the rest of the day following vaccination, despite the medication. Allow extra time for the pre-medications to be in effect before vaccinations are given. Consult your veterinarian regarding his/her recommendation.


Keep in mind that vaccines given within two weeks of each other can interfere with each other so you do not want to separate vaccines by less than a two week period but separating vaccine can mean your pet has less stimulation to deal with at one time. Breaking up vaccines also helps determine which vaccine is the culprit. Spread them out by at least 3-4 weeks if possible.


Vaccination clinics are designed to provide streamlined service for healthy pets in need of their regular immunizations. These clinics are generally not prepared for the individual attention required by a pet who has a history of vaccination reaction. Many clinics are mobile and have moved on the next site by the time a pet has started showing reaction symptoms.  A pet with a vaccine reaction history should receive some special attention in the form of observation in the hospital and/or pre-treatment with anti-inflammatory medication as described above. It is probably best to utilize your primary care veterinarian if your pet has special vaccination needs.


Vaccination reactions severe enough to produce shock are EXTREMELY rare and are a function of an individual pet's immune response. If a pet has suffered a severe or life-threatening reaction in the past, it is not worth continuing to expose the pet to the same allergen again. Vaccination is an important part of responsible pet ownership and should not be omitted without specific veterinary guidance.

Every hospital has its own policy regarding what vaccines are recommended for dogs and cats in general and what vaccines are required for boarding or grooming and most communities have legal requirements for rabies vaccination and may or may not allow for exemption. Vaccine recommendations differ regionally and according to the pet's lifestyle as well as according to the philosophy of the veterinarian in charge of defining a particular hospital's policies.

To view our hospital's vaccination policy click here.


The leptospirosis portion of the DHLPP (distemper) vaccine for dogs has a reputation for being the most likely portion to cause vaccine reactions. This is largely because of the huge size of the inactivated organism which is included in the vaccine and partly related to the bovine serum albumin (a cow blood protein) used in the vaccine's production. (Extraneous proteins create unnecessary immune stimulation and predispose to reaction.) A traditional technique in avoiding vaccination reactions is to avoid leptospirosis vaccine, giving the rest of the Distemper combination vaccine without it.

That said, in a recent study of over one million dogs, the leptospirosis vaccine was not found to be more likely cause a reaction when compared to other vaccines. This may be because of improved vaccine technology. Using only portions of the organism rather than the entire bacterium allows for less exposure to extraneous proteins. Further, newer methods have substantially reduced the amount of bovine serum albumin used.

How important vaccination for leptospirosis is has become a matter of some controversy. The infection is serious, contagious to humans, and exposure generally involves urine contamination of outdoor puddles of water by wildlife or other dogs. Most dogs that become infected, however, are exposed in their own back yards. Some hospitals include leptospirosis in their protocol as is tradition while others omit it. Consider what infections your pet is actually going to be at risk for and avoid unnecessary vaccination and discuss concerns with your veterinarian.


The make up of a vaccine includes an infectious organism rendered harmless but still capable of stimulating the immune system. In some instances the organism is "live" but weakened in some way. In other instances, the organism is killed. Some vaccine is made from killed bacterial organisms rather than viral organisms and other vaccine (such as the rattlesnake bite vaccine) is made against proteins and no infectious organism is involved. The larger the organism and the more complicated the proteins, the more local inflammation results after the injection and this sometimes translates 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. These are not allergic reactions but are inflammatory reactions caused by the immune stimulation of the injection. If a lump persists longer than 3 months from the time of vaccination or is >2 cm in diameter any time after vaccination then it should be removed and biopsied. This is particularly true for cats as they have the potential to develop injection site tumors.



The USDA (United States Department of Agriculture) is responsible for licensing animal vaccines. The agency within the USDA that is immediately responsible is the Center for Veterinary Biologics. This agency is responsible for reviewing vaccine reaction reports and considers whether or not the manufacturer must perform additional investigation. The CVB is also responsible for unannounced testing of vaccine-producing facilities. Vaccination reactions may be reported to the CVB directly or to the manufacturer, though the manufacturer is not required to pass vaccine reaction reports along to the CVB. The advantage of reporting reactions to the manufacturer instead of the CVB is that often the manufacturer will cover the costs of treatment as long as the reaction extends beyond the mild malaise that is a natural result of the vaccination process. Different manufacturers have different policies regarding reimbursement but if one finds oneself with a significant expense from a vaccine reaction, it may be worthwhile to have one's veterinarian look into reimbursement from the manufacturer. To report a vaccine reaction to Center for Veterinary Biologics visit:


Page last updated: 5/18/2019