Pet Boarding & Daycare

How Do Vaccines Work?

How Do Vaccines Work?

By Professional Pet Boarding Council

All pet care facilities should have a strict vaccine policy in place—it is paramount to the safety of the pets in your care, your staff and your business itself. With the ever-changing world of infection–causing organisms, your list of vaccine requirements is likely changing as well. Area veterinarians should continually be consulted about their current recommendations, both for the vaccines required in your facility and their frequency of administration.

Vaccination (immunization) involves intentionally exposing a person or animal to a milder form of an infectious agent, stimulating the immune system to form antibodies against the agent. Then if the person or animal is later exposed to the natural infectious agent, the immune system can respond rapidly to prevent illness.1

Killed Virus, MLV & Recombinant

Vaccines rely on three main techniques to modify the infectious agent into a milder form that will stimulate the immune system but not cause illness. Some, like all canine rabies vaccines, are a killed virus. The virus particles in the vaccine are dead so they cannot cause rabies infection, but the immune system will still “notice” them and form antibodies. Killed vaccines usually have added adjuvant, which is a chemical that “irritates” the immune system to boost the response to the infectious agent.

Other vaccines, like most canine parvovirus vaccines, are a modified live virus (MLV). In MLV vaccines, the virus is live and capable of multiplying in the body, but has been changed so that it does not cause symptoms.

The third type of vaccine is called recombinant. Some Lyme vaccines and some feline rabies vaccines contain the genetic material of only a piece of the virus, not the whole virus. The immune system will recognize the viral piece and form antibodies against it, but because it is only a piece of the virus no infection is possible.

Vaccine Administration

Most vaccines are given by subcutaneous injection (injection under the skin). Some vaccines, particularly canine Bordetella and some feline upper respiratory vaccines, are given by intranasal (in the nostrils), conjunctival (eye drops) or intraoral (in the mouth) routes. It is important that vaccines be given in the appropriate location. For example, if a killed injectable Bordetella vaccine is given intranasally, it may be ineffective, and if a MLV nasal Bordetella vaccine is injected, it can cause a severe reaction.

Although some vaccines are available over-the-counter at feed stores or online, their use is not recommended as there is no guarantee that the vaccine has been transported and stored properly. The pet care facility should only accept pets with proof of vaccines administered by a veterinarian.

Puppies & Kittens

Young animals’ immune systems are immature when they are born. A neonate relies entirely on antibodies that it receives from its mother through the colostrum (the first milk produced after birth). Colostrum is full of antibodies that will be absorbed into the puppy or kitten’s bloodstream to protect it until its own immune system starts manufacturing antibodies. The level of maternal antibodies decreases over time (usually a matter of weeks to months). Initially, when the level is high, vaccinations won’t be effective because the antibodies will deactivate the vaccine. 

Because of this, puppies and kittens (and children) need a series of vaccines. We are trying to catch them in a window of time when the maternal antibodies have fallen enough to allow effective vaccination, but not leave them unprotected against natural infection. Generally puppy and kitten vaccines are given at approximately 8, 12 and 16 weeks of age.


A titer is an indicator of the level of antibodies in the bloodstream. The titer is generally high after the initial series of vaccines then decreases over time. Eventually, the level will fall so low that the dog or cat is no longer immune—it will be at risk of infection. Unfortunately, how long this may take cannot be predicted. Some pet care facilities will accept a titer test result in lieu of vaccination; but because we don’t know how long it will take an antibody titer to fall, annual titer tests are recommended.

Boosters are vaccines which are administered periodically after the initial vaccine series. For example, the immunity induced by the tetanus vaccine in humans only lasts around 5–10 years, so boosters are recommended every 5–10 years. Similarly, boosters are recommended periodically for our pets (for example, boosters every 1 to 3 years for canine parvovirus).

There may be cases when a particular dog or cat is unable to receive a vaccine because of circumstances such as prior severe allergic reactions, certain autoimmune conditions (when the immune system targets the body’s own cells), cancer or other debilitating diseases.

Other than jurisdiction–mandated vaccines (rabies), a pet care facility owner or manager will make the final decision on whether or not to allow the pet to enter the facility. Some factors they will consider are risks to that pet, the general population and employees. 

For more information on Professional Pet Boarding Certification, or to enroll, visit:

1. “Vaccine FAQ and General Information,” Veterinary Partner,