Subscribe to our Newsletter !

Logo AP 2

What Issues You Need to Consider Before You Vaccinate Your Dog or Cat

The first most important thing you should be aware of is that vaccination protects not only your dog, but also the community of animals that lives around.

As vaccination is a contribution to the general health conditions, you should not decide to vaccinate or not only on the basis of the risk your pet runs.

Some vaccines should be given to any dog (see Core vaccines for dogs) or to any cat (see Core vaccines for cats).

Other vaccinations depend on the situation of your pet (see Non-core Vaccines for Dogs and Non-Core Vaccines for Cats).

This guide is designed to help you understand what vaccines your pet really needs.

You will learn in this chapter:

  • How vaccination works
  • When a pet is immunized or at risk
  • How your pet is vaccinated (type of vaccines, site of injection, vaccination schedule)
  • How to deal with special situations (youngest or oldest, animal collectivities)

No doubt that vaccination is a major medical breakthrough. Maybe the most important one !

As recently as a century ago, infant mortality was reaching a frightening 20%. Newborns were often infected with smallpox, diphtheria, measles or pertussis.

Nowadays, the situation improved dramatically thanks to the widespread of complete vaccination programs starting at the youngest age.

Vaccines are very potent. They protect vaccinated people and prevent epidemic outbreaks. They can even eradicate infectious diseases in some areas.

Vaccination is also a major part of veterinary medicine. There is no reason why animals should not benefit of this protection, all the more so that they were the subjects of the first experiments.

Every once in a while epizootics occur in farm animals herds, causing major economic damage and sometimes public health concerns.

Dogs and cats need to get vaccinated too. This is the main reason for their first visit to the vet who generally strongly advises to vaccinate young or unvaccinated pets. The owner seeks medical advice from the vet and decides what type of vaccine will be administered.


A brief history of vaccination

Until recently the development of vaccines followed an empirical approach. Some decades ago, biologists did not have the same level understanding of the immune system as we have today.

The story starts in the 15th century in China with a method called variolation. It consisted of inoculating via the nasal mucosa mildly virulent strains of smallpox.

This process was considered as a fairy tale in Europe. Nevertheless, in 1796, the English doctor Jenner decided to conduct an experiment to demonstrate its reality. He inoculated cowpox instead of smallpox to some patients. Cowpox is a serious disease for bovines, but causes only mild symptoms to humans.

Dr Jenner’s patients became then unaffected by any contamination with smallpox.They were “immunized” although the concept didn’t exist at that time.

Dr Jenner called the cowpox Variolae vaccinae. This is the origin of the term “vaccination”

A century later, Louis Pasteur took the concept to a new level. He theorized the notion of attenuation for live vaccines and created the first vaccines against rabies and anthrax.

First inactivated (killed) vaccines were also designed at the very end of the 19thcentury, by Pasteur and other teams from the US, Germany and UK.

By the 1940’s, biologists understood that they could attenuate a virulent strain bymaking it replicate in cell cultures (Cell culture). They have been extensively using this method for producing most live vaccines.

Since then,the development of vaccine technologies has been accelerating a lot along with a better understanding of the immune system. Searchers are now looking for purified antigens via genetic recombination.


How do vaccines work

The role of vaccination is to mimic a first infection that triggers the primary adaptive immune response which is known to be rather slow and weak.

As a result a next and real infection will induce the secondary adaptive immune response which is both very rapid and potent: the organism is now immunized against a specific pathogen. This is what we were looking for.

See how the immune system works in Chapter 1 : The Immune System.

This is the theory. Let’s now see how to put it into practice.

First step: antigen generation

Most often, it consists of the replication of the bacterium (in cell culture medium) or the virus (in chicken eggs or embryos). Fragments of the pathogen may also be cultivated in cell cultures, bacteria or yeast.

Second step: antigen release, separation and purification

The antigens produced in step 1 need to be separated from the medium that was used for cell growth. Many different methods are used, including filtration, chemical reactions and chromatography.

Third step: pharmaceutical production

Antigens are combined with various ingredients that transform the antigens solution in a ready to use vaccine: adjuvants to enhance the inflammation and the immune response, stabilizers, preservatives…Vaccines are then packed in individual doses.

Fourth step: administration

The objective is still to generate the strongest immunological reaction possible. This is why vaccines are always administered within the peripheral regions of the body: on the nasal and oral mucosa, under the skin, or in the muscles.

This has many benefits. First, this is where the antigen presenting cells (APC: macrophages, dendritic cells) are the most numerous.

Second, these sites of administration are far from the main blood flow routes and the inflammation generated by the injection and enhanced by the adjuvant fades slower. It gives extra time to the APCs to harvest antigens.

Dendritic cells and macrophages transport the antigen or antigen fragments to lymphocytes (B-cells, T-cells, T-helpers) to initiate a strong immune response and the creation of memory cells.

Important warning 1: vaccination is very specific to the antigen which is administered. If the antigen is different from those of the real infection, the vaccination does not work.

Important warning 2: the duration of the immunity depends on the intensity of the immune response generated by the vaccine.


Types of vaccines

Live attenuated vaccines

In live attenuated vaccines viruses or bacteria are still alive. They behave the same way as in a real infection. But their virulence, i.e. their ability to induce a disease, has been considerably altered.

Historically, the attenuation was obtained by repeated culture. Nowadays, wild pathogens are genetically modified.

Live vaccines are the only vaccines that trigger both the humoral and the cellular adaptive immune response. As a consequence, they are more effective than other types of vaccines. The immunization they induce lasts longer and boosters are less frequently required.

There are some drawbacks though. Cold chain integrity should be maintained from the production until the administration to a patient. This means live vaccines need to be stored in refrigerators and transported in refrigerated trucks.

In some rare occasions, the virus or the bacterium reverts back to its wild, virulent form. It may induce the symptoms of the disease it is supposed to protect us from. The symptoms are usually mild. But it is therefore not recommended to use live vaccines on immunocompromised patients.

Non live-vaccines

They are made of either of full inactivated pathogens, fragments of them, selected antigens from them or toxins produced by them.

Of course, because they are not alive, they can’t replicate. Thus, the vaccination can’t mimic a real infection. It is not able to trigger the immune system as efficiently as a live vaccine, or at least to induce a cell mediated immune response.

In all non-live vaccines manufacturers include adjuvants to enhance their immunogenicity. The intended effect is that the adjuvants hold back the inactivated germs or antigens within the site of administration and release them progressively over time. It increases the exposure time of the antigens to the organism and therefore the quality of the immune response.

Non-live vaccines also require one or more boosters to reinforce the immunization.

On the plus side, killed or inactivated vaccines induce less adverse events except at the site of injection.

Inactivated or killed vaccines

They are made of full viruses (inactivated vaccines) or bacteria (killed vaccines) inactivated by heat or chemicals.

The structure of the germ remains intact and therefore presents all its original antigens to the organism. But, it can’t replicate and mimic a real infection.

Toxoid vaccines

These vaccines are not aimed at controlling the pathogenic germ but at protecting against the toxins it produces.

Toxins are extracted and then purified from the pathogen culture. They are then administered to the patient with some excipients. The immune response consists of the production of antibodies directed against the toxin’s antigens.

Polysaccharide and conjugate vaccines

These vaccines target the antigens of the capsule of encapsulated bacteria as, for instance, Streptococcus. These capsules are mainly made of polysaccharides.

The issue here is that polysaccharides send a low signal to the immune system. They do not activate T-cells and especially helper T-cells. The overall immune response is weak. Frequent boosters are needed to maintain protection.

A conjugate vaccine is the combination of a polysaccharide vaccine and a protein carrier. The protein carrier is selected for its high immunogenic capability, especially with regards to T-cells.

Compared to polysaccharide vaccines, conjugate vaccines are a big step ahead in term of the strength of the immune response they generate and duration of protection.

Recombinant vaccines

Recombinant vaccines involve the selection of one or several antigens from the pathogen.

The gene(s)coding for this(ese) antigen(s) are then inserted into a vector, generally a virus. There are 2 possibilities:

The vector is injected into the patient

Or the vector serves as antigens producer and the antigens are injected into the patient.

This is advanced technology. The challenge is to select the antigens that elicit a strong immune response and are very specific to the bacteria/virus they come from.

The recombinant vaccine strategy gives more control on the development and production of the vaccine. It can target several pathogens at a time and it induces much less adverse events than attenuated live vaccines.

Although recombinant vaccines can also trigger T-cells immune response, they need strong adjuvants and multiple injections to ensure a good protection.


Routes and sites of administration

Vaccine designers try to elicit the strongest immune reaction possible while avoiding adverse events.

This is why vaccines are always administered in the peripheral regions of the organism: close to the skin, to the mucous membranes or in the peripheral muscles. Far from the large arteries, the inflammation lasts longer there and the immune system has got time to build a stronger response.

Intramuscular (IM)

This is the most frequent and easier way to administer a vaccine. The vaccine is injected in muscular tissues. All IM vaccines include an adjuvant.

Subcuteneous (SC)

The injection site is located just under the skin.

It seems that this type of injection generates more adverse events on the site of injection.

Intradermal (ID)

The injection site is situated in the top layer of the skin. Because the skin is thin, the injection needs to be very precise, which makes the administration a bit difficult.


This is the easiest route of administration. No needle needed!

Oral vaccines trigger the production of IgA antibodies, known for their protective effect against mucosal infections.


Nasal vaccines follow the natural way of respiratory infections. Nasal vaccines also elicit both the mucosal and the systemic immune systems.

They are easy to administer to human patients.


Passive immunization

Passive immunization is the transmission of active antibodies to a non-immune individual.

Natural passive immunization occurs when a mother transfers her antibodies to the fetus in the last third of pregnancy in utero (IgG antibodies), or to her newborn babies through her milk (IgA antibodies).

This protection is very useful since newborns have a weak immune system and rely almost exclusively on the defenses provided by the mother. For instance, premature babies have a higher risk of contracting an infection.

Passive immunization is also a therapeutic weapon for the patients in need of an urgent solution: either because of a current infection or an immunodeficiency. They will be injected with blood of serum from an immune donor (human or animal) containing the specific antibodies for the pathogen. This is a short term medical measure.

For example, it is the case for preventing the infection by rabies after the bite of an infected animal.

Cell-mediated immunity is difficult/impossible to transmit. Donor’s T-cells would consider the host’s cells as foreign bodies (Non-Self) and start attacking them. It could induce some serious disorders.


How to assess vaccine efficacy

Vaccination failure

There are 3 main causes of vaccination failure: maternal antibodies, poor immunogenic vaccine, and poor responding patients.

Maternal antibodies

During the pregnancy the mother transmits her antibodies to her fetus in utero, via the placenta. Later on, she keeps on giving some to her newborn via her milk.

These antibodies necessarily interfere with the vaccines administered to her puppy or kitten. Maternal antibodies “attack” the vaccines' antigens and neutralize them before the puppy’s or kitten’s body has got the time to build up its own immunity.

Maternal antibodies gradually disappear from the animal’s blood within a few months.

Some puppies (or kittens) can respond to a vaccination as early as 6 weeks of age, whereas others will have to wait until they are 16 weeks.

This why your vet will recommend a vaccination starting at 6-8 weeks and then every 2-4 weeks until 16 weeks of age or older.

In special situations where the vaccination is limited to one single dose, the vaccine should be administered not sooner than 16 weeks.

Poor vaccine immunogenicity

Vaccines manufacturing standards are high nowadays. Vaccines are a large market for pharmaceutical companies and vaccines are made in large facilities that strictly follow legal manufacturing requirements. In addition, once produced, vaccines are tested.

There may be some issues though. Live attenuated vaccines require to be kept at low temperature. Rarely, there are some unnoticed accidents during storage or transportation that inactivate totally or partially this living material.

Some inactivated/killed vaccines have naturally a poor immunogenicity and sometimes fail to initiate an immune response.

Poor animal response

Young dogs or cats on one hand, and aging pets on the other hand have a weaker immune system. They may respond poorly to immunization attempts.

A study (Kennedy et al. 2007) showed that smaller breeds have a better and more durable antibodies response than larger ones. They should respond better to vaccination.

As immune response is genetically determined, there should be some differences from one breed to another or from one individual to another.

Serological testing

Your vet may propose you to perform a serological test on your dog or cat

Serological testing measures the quantity of antibodies circulating in the blood for an antigen or infectious disease.

It is a way to assess if the vaccination was successful or if the animal is still immunized.

It is not perfect, though.

If the test is positive (i.e. there are enough antibodies circulating in the blood), it’s OK. Everything’s fine. The animal is immunized.

However, a negative test doesn’t mean that the animal is not immunized and because of 2 reasons:

  • There may still be numerous memory B-cells that can transform very quickly in antibodies releasing plasma cells
  • Cell-mediated immunity is not measured with antibodies titers

The use of these tests is questionable since they are expensive: they cost more than the vaccination itself.

Nevertheless, they may prevent from unnecessary vaccination.


Adapting vaccination to the age

How to vaccinate young puppies or kittens

Without the maternal antibodies, a newborn would be the prey of numerous infections.

In the first weeks of life,the immune system of puppies and kittens is far from being mature:

  • Effector B-cell takes more time to differentiate and proliferate
  • Less antibodies and memory cells are produced
  • Plasma cells produce short-lived antibodies

It makes vaccinating a puppy or a kitten a bit tricky. On one hand, the immature immune system does not react strongly. On the other hand, the maternal antibodies neutralize a part of the vaccine antigens.

This is why vaccinating a puppy or a kitten requires several injections. It gives the immune system more exposure to the pathogen’s antigens. It also gives more opportunities to admnister the vaccine at the precise point in time when the immune system is mature enough to create an appropriate response and when maternal antibodies have disappeared.

How to vaccinate adult dogs or cats

Dogs that were successfully vaccinated with live attenuated vaccines do not need to be revaccinated before three years.

As there is some uncertainty in whether they were immunized properly when they were young puppies, your vet may recommend to vaccinate them again at 6 months or a year, and then every 3 years.

Vaccination against rabies is often required by law. You should refer to the regulation in your state or country and stick to it.

How to vaccinate aging dogs or cats

As for all organs, the immune system gradually weakens over time. This deterioration accelerates at the end of life.

There is an overall and marked decrease in the number of all types of lymphocytes. Antibodies are fewer and develop a lower affinity for antigens. The immune system looses part of its reactivity against new infections. Furthermore, it fails to maintain immunization overtime.

As a result, aging pets become more susceptible to infections and need boosters more often.

The issue is that this preventative measure is often overlooked. Vaccination schedule should be recalled at each geriatrics visit to the vet.


Vaccination in a shelter

In a shelter, a kennel or a cattery, infections can spread rapidly. Animals come in or out very frequently. Some of them may have a poor health status or carry infectious diseases.

Shelter managers need to prevent the onset of epidemics in their facilities. They have to ensure that any pet joining the group do not carry any serious infective pathogen.

They will first look at the pet’s medical record and search for the vaccinations it got previously. And then make it vaccinated if needed.

In the case of puppies or kittens the WSAVA recommendations are reinforced due to the higher risk of being infected. Young pets should get their first injection at 4-6 weeks and then every 2 weeks until the age of 20 weeks.


Duration of immunity

From a medical standpoint, it is the period for which a pet is protected from the infection it is vaccinated for.

Usually, the information you get in the vaccine's leaflet is a minimum time, because clinical studies may have not been continued long enough.

Recent experiments (Schultz 2006) indicate that dog or cats vaccinated with live vaccines are immunized for at least 3 years.

Inactivated vaccines induce a weaker immune response and vaccination. In this case, they are recommended every year and sometimes more frequently.