Vaccination: What Are We Actually Protecting Against?

Vaccination is often reduced to a simple question: “Has my dog had its vaccinations?” But vaccination is not really about the injection itself. It is about preparing the immune system to recognise particular infectious threats before the dog encounters them.

When we talk about vaccination in dogs, we are not talking about one single disease or one universal vaccine. Different vaccines target different pathogens, and not every vaccine is appropriate for every dog. Some are considered core because they protect against serious diseases with broad distribution. Others are non-core because their usefulness depends more heavily on factors such as geography, environment, lifestyle and exposure.

Understanding what we are actually vaccinating against makes it much easier to understand why vaccination protocols differ between dogs and why simply counting injections doesn't tell the whole story.

The core C3 vaccines

For dogs, the core C3 vaccination protects against three important viral diseases: canine parvovirus, canine distemper virus and canine adenovirus.

These diseases can cause severe illness and death, particularly in puppies, which is why protection against them is considered fundamental. The WSAVA (World Small Animal Veterinary Association) 2024 vaccination guidelines classify canine distemper virus, canine adenovirus and canine parvovirus as core vaccines for dogs. The Australian Veterinary Association also supports core vaccination against these diseases.

Although these three viruses are routinely discussed together as “C3”, they cause very different diseases.

Canine parvovirus

Canine parvovirus is highly contagious and particularly dangerous to young, unvaccinated or incompletely vaccinated dogs.

The virus has an affinity for rapidly dividing cells. In puppies, this includes cells lining the gastrointestinal tract and cells involved in blood-cell production in the bone marrow. Damage to the intestinal lining can cause severe vomiting and diarrhoea, dehydration, lethargy and loss of appetite. Bone marrow involvement can reduce white blood cell production, compromising the puppy's ability to respond to infection. Severe disease can therefore involve much more than gastrointestinal upset.

Parvovirus is also environmentally persistent. Large quantities of virus can be shed in the faeces of infected dogs, and contaminated environments and objects can contribute to transmission. A puppy therefore does not necessarily need direct contact with a visibly sick dog to encounter the virus.

Vaccination reduces the risk of severe disease following exposure. At a population level, widespread vaccination also reduces the number of susceptible dogs through which the virus can circulate.

Canine distemper virus

Canine distemper is a highly infectious systemic viral disease. It can affect the respiratory, gastrointestinal, immune and nervous systems.

Early signs can include fever, lethargy, loss of appetite, ocular and nasal discharge, respiratory disease, vomiting and diarrhoea. Neurological disease can develop later and may include muscle twitching, seizures and other neurological abnormalities.

The severity of disease varies between infected dogs, but serious neurological complications can occur. There is no simple treatment that reliably eliminates the virus once significant disease has developed, making prevention particularly important.

Canine adenovirus

Canine adenovirus is slightly more complicated because there are different canine adenoviruses with different disease associations.

Canine adenovirus type 1, or CAV-1, causes infectious canine hepatitis. This can be a severe disease affecting the liver, blood vessels and other organs, and severe cases can be fatal.

Modern canine core vaccines generally use canine adenovirus type 2, or CAV-2, rather than CAV-1. CAV-2 is sufficiently related antigenically to CAV-1 to provide protection against infectious canine hepatitis while avoiding some of the adverse effects historically associated with CAV-1 vaccination. CAV-2 is also associated with respiratory disease.

So when CAV-2 appears on the label of a C3 vaccine, it should not simply be interpreted as a vaccine against “kennel cough”. Its inclusion provides core protection against canine adenovirus, including the disease caused by CAV-1.

But vaccination doesn't stop with C3

C3 is the core vaccination, but dogs can encounter many other infectious organisms.

Some additional vaccines are considered non-core. This does not mean that the diseases they target are unimportant. It means that the risk is more dependent on where the dog lives, what it does and what it is likely to encounter.

The WSAVA and AVA frameworks distinguish between core, non-core and vaccines that are not recommended because the evidence does not justify their routine use. For dogs, examples of non-core vaccination include protection against canine parainfluenza virus, Bordetella bronchiseptica and Leptospira.

This distinction is important because it means there is no scientifically sound reason to assume that every dog should receive exactly the same collection of vaccines throughout its life.

Canine infectious respiratory disease

“Kennel cough” is often spoken about as though it were a single disease caused by a single organism. It isn't.

Canine infectious respiratory disease is a syndrome that can involve several infectious agents, including canine parainfluenza virus and Bordetella bronchiseptica, as well as other viruses and bacteria.

Vaccination against some of these organisms can be particularly relevant to dogs that regularly mix with large numbers of other dogs. Boarding kennels, daycare, group training, dog shows and other high-contact environments can increase exposure opportunities.

The important distinction is that these vaccines are addressing a different risk from the core C3 diseases. A dog that rarely encounters groups of unfamiliar dogs may have a very different exposure profile from a dog attending daycare several times a week.

This is why non-core vaccination is generally considered on an individual basis rather than automatically applied to every dog. The AVA specifically states that non-core vaccines should be selected according to the individual animal's circumstances and in consultation between the veterinarian and owner.

Leptospirosis

Leptospirosis is another important example because it is fundamentally different from the diseases covered by C3.

Leptospira are bacteria rather than viruses. Infection can occur when dogs are exposed to environments contaminated with the urine of infected animals, with exposure influenced by factors such as geography, wildlife, water and environmental conditions.

Leptospirosis can cause serious systemic disease, including kidney and liver damage, and some strains can also infect humans. This makes it an important disease from both veterinary and public-health perspectives.

However, the risk is not identical everywhere. That is precisely why leptospirosis vaccination is considered non-core rather than automatically appropriate for every dog.

The decision needs to take the individual dog's environment and likely exposure into account. In areas where leptospirosis occurs or where environmental exposure is significant, the risk-benefit calculation may look very different from that of a dog living in an area with little recognised risk. The WSAVA guidelines likewise classify Leptospira vaccination as non-core and recommend considering it according to geographical and individual risk.

Why isn't every vaccine considered core?

This is an important question because “core” does not mean “the only vaccine that matters”.

Core vaccines are intended to protect all dogs against severe diseases with broad distribution and significant consequences. Non-core vaccines are aimed at diseases where exposure varies substantially between locations, lifestyles or individual circumstances.

Imagine two dogs.

One spends most of its life at home, walks locally and has little contact with groups of unfamiliar dogs. Another regularly attends daycare, boarding facilities, group training and other high-contact environments.

Their exposure profiles are different.

Likewise, a dog living in an area where a particular pathogen is established may face a different risk from one living somewhere that disease is rarely encountered.

The purpose of a risk-based vaccination program is therefore not to minimise vaccination or maximise vaccination. It is to match protection to actual risk.

What does the immune system actually learn from a vaccine?

A vaccine does not simply place a protective substance into the bloodstream. It gives the immune system information.

Vaccines expose the immune system to an antigen in a form designed to stimulate an immune response without producing the disease the vaccine is intended to prevent. Depending on the vaccine, this may involve a modified-live organism, an inactivated organism, a component of the pathogen or another form of antigen.

The immune system responds through several mechanisms, including the production of antibodies and the development of immunological memory. If the vaccinated animal subsequently encounters the relevant pathogen, its immune system can recognise it and respond more rapidly than it would during a first encounter.

Vaccination is therefore better understood as immune preparation rather than an invisible protective barrier.

And this is an important distinction: being vaccinated and being immune are related, but they are not interchangeable terms.

Why puppies are different

Puppy vaccination presents an additional challenge because puppies receive antibodies from their mothers.

These maternally derived antibodies, commonly called MDA, are acquired primarily through colostrum during the first hours after birth. They provide important early protection while the puppy's own immune system is developing.

But the same antibodies that can protect a puppy against infection can also interfere with vaccination. If enough maternal antibody remains when a vaccine is administered, it can neutralise the vaccine antigen before the puppy develops an adequate active immune response.

Maternal antibody levels do not disappear at exactly the same age in every puppy. The amount received and the rate at which it declines vary between individuals.

This creates a period in which a puppy may have enough maternal antibody to interfere with vaccination but not necessarily enough to provide reliable protection against exposure.

This is one of the reasons puppy vaccination involves a series of doses rather than relying on a single early vaccination.

The WSAVA 2024 guidelines recommend core vaccines every three to four weeks in puppies, with the final puppy vaccination at 16 weeks of age or older in most circumstances. The AVA similarly recommends puppy core vaccination beginning at six to eight weeks, followed by vaccination every two to four weeks, with the final dose no earlier than 16 weeks.

Why the timing of the final puppy vaccination matters

It can be tempting to think of puppy vaccination as a simple numbers game: one vaccination plus another plus another equals protection.

Immunology is not quite that simple.

A vaccine administered at eight weeks and one administered at sixteen weeks are being given in different biological circumstances. The amount of maternal antibody present may be very different, and therefore the likelihood that the vaccine can successfully stimulate active immunity may also be different.

This is why the age of the final puppy vaccination matters, rather than simply counting how many injections have been given.

There is an additional complication in Australia: some registered canine vaccines have product-specific schedules that allow the primary course to be completed earlier than 16 weeks. The existence of such a registered label claim does not mean that maternal-antibody interference has ceased to be biologically relevant. It means that the particular product has evidence supporting its approved claims and schedule.

The APVMA requires vaccine efficacy claims and administration schedules to be supported by appropriate data, including laboratory and, where appropriate, field studies. Approved vaccine labels specify the claims, onset and duration of immunity and directions for use for that particular product.

That distinction is important. A registered early-finish schedule is not the same thing as a guarantee that every puppy will be immune at the moment its final dose is given.

Why vaccination does not guarantee immunity

This is perhaps one of the most important things for owners to understand.

A vaccination is an intervention designed to induce protective immunity. It is not proof that an individual puppy has become immune.

The APVMA requires efficacy claims for veterinary vaccines to be supported by data from laboratory and/or field studies in the target species. Importantly, vaccine efficacy can be defined in different ways. A product may have evidence supporting prevention of mortality, reduction in clinical signs, prevention or reduction of infection, or other specific outcomes. The onset and duration of immunity are also part of the evidence supporting approved claims.

Therefore, a statement such as “this vaccine prevents the disease” needs to be understood in the context of the specific claim and evidence supporting that product.

It does not mean that every vaccinated animal will respond identically, that exposure can never occur, or that infection is biologically impossible.

Vaccination substantially improves the likelihood of protection. It does not create an absolute guarantee.

C3 vaccines are not necessarily identical

Another important point is that C3 is a description of the diseases being targeted, not a description of one universal vaccine formulation.

Different products can contain different vaccine strains, formulations and other characteristics. They can also have different registered vaccination schedules and different evidence supporting their particular claims.

This means it is not scientifically useful to compare two C3 vaccines simply by saying that one has “more” of something and therefore must be stronger or better.

Vaccine performance depends on much more than a single number. The vaccine strain, formulation, immune response, vaccination schedule, onset of immunity, duration of immunity and evidence supporting the product's claims all matter.

The APVMA's regulatory framework reflects this by assessing efficacy claims, schedules and other product characteristics for individual veterinary vaccines.

Vaccination is only one part of disease prevention

Vaccination is an important disease-prevention tool, but it is not the only one.

A dog's infectious-disease risk is also influenced by environmental contamination, local disease prevalence, contact with other animals, hygiene and the health status of animals it encounters.

This becomes particularly important during puppyhood.

Protecting a puppy from infectious disease does not mean isolating it from the world until its vaccination course is complete. Sensible disease prevention and appropriate socialisation can occur together.

The AVA recognises that properly conducted puppy preschool in a clean environment can be undertaken before a puppy has completed its full vaccination course.

The objective is to manage exposure rather than eliminate all exposure. A puppy can experience people, sounds, surfaces, environments and everyday situations while avoiding unnecessarily high-risk contact with potentially contaminated environments or unknown animals.

So, what are we actually protecting against?

Ultimately, vaccination is not about ticking a box on a veterinary record.

We are protecting dogs against infectious diseases that can cause severe gastrointestinal disease, respiratory illness, systemic infection, neurological disease, liver and kidney damage and death.

For puppies, we are doing this during a particularly complicated period of immune development, when maternal antibodies are gradually declining and the puppy's own immune system is learning to respond to the world around it.

But vaccination is not one-size-fits-all.

C3 protects against three important core viral diseases. Other vaccines address other pathogens and become relevant according to the individual dog's risk. A dog regularly mixing with large numbers of other dogs may have a different respiratory-disease exposure profile from a dog living a quieter lifestyle. A dog living in an area where leptospirosis is a meaningful risk may have a different vaccination decision from one living somewhere that exposure is unlikely.

That is why the useful vaccination questions are bigger than “Has my dog been vaccinated?”

What disease are we vaccinating against? How serious is that disease? How likely is this particular dog to encounter it? What vaccine is being used? What does the evidence show about its efficacy? What schedule is registered for that product? When does protection begin? How long does it last? And how does the dog's individual environment alter the risk?

These questions move vaccination away from simply counting injections and towards understanding what those injections are actually designed to achieve.

Vaccination is not a guarantee against every infectious disease. Nor does every dog require exactly the same vaccines throughout its life.

It is a way of preparing the immune system to respond to specific threats, using evidence to decide which threats matter for a particular dog and which vaccination strategy provides appropriate protection.

And that is what we are actually vaccinating for.