“Vaccine Reaction”? Probably Not.

September 27, 2023

Vaccinations are a scientific miracle. The number of human (and animal) lives saved by vaccinations cannot be overestimated. Once-common diseases such as canine distemper and feline panleukopenia have been virtually eliminated in vaccinated pets (I’ve only seen a handful of pets with these diseases, as opposed to some of my more seasoned colleagues, who used to diagnose and treat these diseases with frequency.)


Regular vaccinations for preventable diseases such as parvovirus, distemper, panleukopenia and rabies are recommended for all kittens and puppies, at regular intervals from 8-16 weeks of age, and then again at one year of age. The interval for re-vaccination in adult pets is dependent on the lifestyle of your animal, and conversations with your veterinarian. The rabies vaccine is required by law for cats and dogs in Pennsylvania, and thus is a non-negotiable core vaccine.

One of the most common concerns I hear from clients in regards to vaccination is “Fido had a reaction to XYZ vaccine when he was a puppy, so we don’t want to give him that vaccine any more.” Or “Fluffy was so lethargic after her vaccines last year, I don’t think she should be vaccinated again.” I certainly appreciate clients’ concerns; no one wants Fido or Fluffy to feel under the weather, especially from something that we “inflicted” upon them.

Here’s how vaccinations work: your pet is injected with a small amount of the viral or bacterial pathogenwe are trying to prevent. The virus has been modified, or killed, so it cannot cause fulminant illness. In response to the presence of this inactivated virus or bacteria, Fido’s immune system creates antibodies, which will help protect him if he is ever naturally exposed to the pathogen. Because vaccines are introducing a very small amount of a microorganism into your pet’s body, it is normal for them to mount an immune response that can result in flu-like symptoms for a short period of time (usually less than 24 hours, if seen at all.) I tell clients all the time “vaccines may make your pet a little bit sick today, so they don’t get a whole lot sicker later.”

Most people can relate to feeling flu-like symptoms after receiving the flu vaccine. Or to having a sore, tender arm after being vaccinated in the deltoid (shoulder) muscle. Some lethargy, mild fever, chills, muscle soreness, and/or a general feeling of “blah”may also be seen post-vaccine.These symptoms are NOT a vaccine reaction, but a completely normal response to vaccination! Any pet owners who also have children may remember how fussy, and feverish and uncomfortable their kids might have been after vaccinationas infants. I remember giving my son Tylenol and holding an ice pack on his thigh, since he was painful after his 3-month vaccines. While these symptoms may be mildly unpleasant, many patients do not exhibit them at all, and those that do are often back to normal within 24 hours.

A vaccine reaction is defined as an allergic reaction to vaccines, and is usually dramatic and sometimes life-threatening. Hives, facial swelling, vomiting, shock, collapse, and labored breathing are some of the symptoms of a true anaphylactic vaccine reaction, and often these symptoms occur within moments of vaccination, typically before the owners even check out from their appointment. These types of reactions are fortunately rare, but can be life-threatening; pets having an allergic or anaphylactic reaction to a vaccine should understandably never be vaccinated for that pathogen again!

Contrast this to the phone calls we get the day after Fido has been vaccinated. “Fido is sleepy, and his appetite is a bit off, and he seems sore where the shots were given yesterday.” Is Fido uncomfortable as a result of his immune system’s normal response to vaccination? Yes! Is he having a “vaccine reaction”? NO! Should we discontinue his vaccines going forward? NO! Is there anything we can do to keep this from happening again? Yes! If pets experience malaise and discomfort post-vaccination, theycan be prescribed a non-steroidal anti-inflammatory to ward off fever, aches and pains. If a patient has a history of vaccine soreness, I can preemptively give them an NSAID each time they are vaccinated. This often helps keep any side effects to a minimum, and it allows us to continue vaccinating Fido regularly as needed.

The phrase “vaccine reaction” is over-used by veterinary staff and clients alike, and it can result in medical misunderstandings. There is a huge difference between an anaphylactic reaction to vaccination, and a normal post-vaccination immune response!