Pet Vaccines

Understanding Vaccines from a Veterinarian's Perspective

An In-Depth Look with Dr. Ingrid Pyka

Cat receives vaccination

Infectious diseases can cause serious illnesses, permanent tissue/organ damage, or even death.

Vaccines are the easiest way to prevent many of these diseases or at least reduce their severity.

How Do Vaccines Work?

Infectious diseases are caused by pathogens, such as bacteria, viruses, or protozoa.

Disease strains can be identified by unique markers called an antigens. Once the immune system discovers an antigen, immune cells can develop a new counter protein called an antibody.  The antibody either inactivates the antigen directly or acts as a signal to other parts of the body’s immune system to fight against this pathogen.

Though each antibody responds only to the specific antigen for which it was made, the body replicates this antibody and maintains a reservoir for future use. Depending on the pathogen, the immune system may sustain and produce this antibody for months, years, even lifelong.

Vaccines actually introduce tiny amounts of antigens into the body, stimulating the production of these protective antibodies. This way, the body’s immune system has already been trained with active antibodies to recognize, mark, and battle these known pathogens. The body is ready to quickly respond if a true exposure to the disease should occur.

So Why Don’t Vaccines Cause Illness?

Veterinarian examining dog

To prevent the vaccination from actually giving the disease to the patient, the antigens have been changed in various ways. In some vaccines, the bacteria or virus is killed. Killed vaccines expose the immune system to the antigenic proteins, allowing appropriate antibody production. But, since they are not alive, they cannot replicate in the body to cause illness.

Modified live vaccines contain changed forms or different strains of the disease agent. The modified viruses or bacteria of these vaccines can reproduce and stimulate the immune system, but are modified so they should not cause disease or illness.

New vaccines recently becoming more available involve recombinant technology. These vaccines utilize the ability to alter and/or copy pieces of the actual antigenic DNA. This way, the immune system can make the necessary antibodies without ever being exposed to the actual pathogen. Recombinant vaccines appear to be the safest and most effective vaccine yet.

Protection gained from vaccines generally far outweighs the potential risk. As benefit vs. risk does vary for each individual animal, discuss with your veterinarian the most appropriate vaccine protocol for each of your pets.

Can Vaccines Harm Your Pet?

Luckily, complications from vaccinations are uncommon. However, as important as vaccines have become, they are not entirely harmless.

Dog sits in chair

The most common side effects noted by owners include fatigue and/or decrease in appetite/attitude during the first 24 hours of a vaccination. These are mild and usually relatively well tolerated.

If, however, within a few hours of receiving a vaccine, you notice your pet developing a swollen face or muzzle, hives, intense itching, vomiting or even collapse, treat this as an emergency. Take your pet to a veterinary hospital immediately! Vaccine reactions can be quite uncomfortable, as well as potentially life threatening.

If a lump develops after a vaccine is given and persists for over a few weeks, it should be evaluated by a veterinarian. In cats, some vaccines have been correlated with causing rare, but aggressive skin tumors known as fibrosarcomas.

Protection gained from vaccines generally far outweighs the potential risk. As benefit vs. risk does vary for each individual animal, discuss with your veterinarian the most appropriate vaccine protocol for each of your pets. 

Why Booster?

During the initial vaccination period, it generally does take at least two consecutive vaccines to stimulate enough antibody production for full protection against a disease.

In newborn animals, vaccines require a longer series of boosters due to the antibodies passed from the mother. Though short-lived, the maternal antibodies in the youngster’s immune system attack the vaccine’s antigens. This dramatically lessens the vaccine’s efficacy. Once the maternal antibodies are gone, however, the puppy or kitten must rely on the vaccine to develop its own long-lasting protective antibodies.

Depending on the disease, the antibodies do dwindle at variable times. A booster serves as a reminder to the body to re-stimulate the production of the appropriate levels of antibodies.

What Are We Vaccinating For?

There are many vaccines out on the market. Core vaccines are those that all pets should receive. Your veterinarian may recommend other non-core vaccines based on where you live and your pet’s lifestyle.

Canine core vaccines:

  • Distemper
  • Adenovirus II
  • Parvo
  • Rabies

Common canine non-core vaccines:

  • Parainfluenza
  • Bordetella
  • Leptospirosis
  • Lyme

Other canine vaccines:

  • Corona
  • Giardia
  • Rattlesnake
  • Peridontal

Feline core vaccines:

  • Feline Viral Rhinotracheitis
  • Feline Calicivirus
  • Panleukopenia
  • Rabies

Common feline non-core vaccines:

  • Feline Leukemia

Other feline vaccines:

  • Feline Coronavirus
  • Chlamydophila Felis
  • Bordetella Bronchiseptica
  • Feline Immunodeficiency Virus

Booster Schedules

You may find large discrepancies as to recommendations for puppy and kitten booster schedules.

In general, initial vaccines for puppies (DA2PP) and kittens (FVRCP) begin at 6-8 weeks of age, boostering every 3-4 weeks until 14-16 weeks old. Adult dogs and cats receiving their first known vaccines should also have a second booster in 3-4 weeks.

The first rabies may be administered between 12-16 weeks, though most veterinarians give the rabies with the last of the puppy/kitten series.

After the initial set of vaccines, both dogs and cats should be re-vaccinated in 1 year, then every 3 years. Most non-core vaccines require annual boostering (Bordetella every 6 months).  

Table 1 – Small Animal Core Vaccination Booster Guidelines- after initial booster series complete*




(Feline Viral Rhinotracheitis, Calicivirus, Panaleukopenia)

3 years


(Feline Leukemia)

1 year


(Distemper, Adenovirus, Parainfluenza, Parvovirus)

3 years


6 months




1 year


1 year (1st vaccine)

3 years (add’l vaccines)

* Based on current guidelines from AAFP (American Association of Feline Practitioners) and AAHA (American Animal Hospital Association). Additional vaccines are available for other species, specific conditions, environments, and geographic regions. Always consult with your veterinarian for the protocol that best suits your pet.

Your Pet

Bring all your pet’s previous records to your veterinarian so that he or she may guide you with the next due date and necessary successive boosters. Remember, even though your pet’s next vaccine may not be due for three years, examinations by a veterinarian are very important.

Schedule physical examinations with your pet’s doctor every year. Healthy pets are happier pets.

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