Vaccine boosters for people are all over the news at the moment. As our understanding of COVID-19 changes, so too do the recommendations for booster jabs. Earlier in the year, no one needed a booster, and now almost everyone is being recommended them. These changing guidelines got us thinking about veterinary vaccinations. Do older dogs need regular boosters? Or is there an alternative to yearly jabs?

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How Do Vaccines Work?

By now we’re sure everyone knows how vaccines work, but just in case, here is a little recap:

Our body is constantly assaulted by bacteria, viruses, fungi, parasites and other organisms that wish to do us harm. To protect us we have an immune system. The innate immune system is non-specific, attacking any hostile cells they find. Think of it like a big shotgun. The acquired immune system is specific. Vaccines use the acquired immune system. Think of it like a sniper rifle: very precise and accurate. 

We have millions of acquired immune cells 

Every one is unique, carrying the key (antibody and cell receptors) to a very specific lock (antigen). Every pathogen carries a different antigen. When a cell carrying the receptor for a specific antigen meets the pathogen carrying that antigen, the receptor and antigen interacts like a key in a lock, enabling the cell to destroy the pathogen. Some cells work by attacking viruses or bacteria inside a cell (T-cells) and others by attacking them outside (B-cells making antibodies), but the principle is identical.

Once the cell has interacted with a pathogen, the body replicates the cell many times

This creates millions of keys for all the hostile locks, so our bodies can fight off the invading pathogen. If successful, the pathogen is defeated and we get over being sick. Our bodies know that our enemies may return, so stores a number of the cells as memory cells. If the pathogen ever invades our body again, memory cells will detect the specific antigens, allowing millions of immune cells to be rapidly produced to defeat the invader before we ever get ill. This is the concept of immunity, and why we only get sick from diseases like chickenpox once. After the first time we are infected, we carry the memory cells needed to fight off the chickenpox virus. So if it ever enters our body we quickly fight it off and do not get ill.

Vaccines utilise immunity in a very similar way

They provide the body with antigens for certain diseases so memory cells can be created for them. After being vaccinated, our body creates the memory cells specific to the actual pathogen. If we are unfortunate enough to encounter the pathogen for real, our body can produce a quick and effective immune response, reducing or even preventing sickness. To avoid the vaccine causing illness, vaccines contain dead or inactivated pathogens. There is very little (though not zero) risk of catching a disease from a vaccine. The MMR vaccine, yearly flu jab and even the COVID-19 vaccine all work in similar ways.

Immunity varies from person to person, and disease to disease

Some memory cells last for years, giving us lifelong protection against a disease. Some fade over time, requiring booster vaccines. The pathogens themselves are also constantly trying to evade our immune systems, changing their locks slightly so the keys don’t fit any more. This is why we need new yearly flu boosters, and why new COVID variants may evade vaccination.

Dog Vaccination Guidelines

Traditionally, dogs have had yearly boosters of all the ‘core’ vaccines. These vary, but generally include distemper, infectious hepatitis, parvovirus and leptospirosis. However, in recent years, it has been recognised that the immunity for some of these vaccines last longer than one year. However, research in a document created by the World Small Animal Veterinary Association (WSAVA) has revealed that this may result in over-vaccination, and these old-fashioned yearly boosters might not always be necessary. A more common protocol now is, therefore, to have distemper, hepatitis and parvovirus (DHP) every three years, with leptospirosis (L) yearly. 

WSAVA Guidelines

The WSAVA put out new guidelines for veterinarians in 2015. A number of novel ideas and theories came out regarding vaccination. Most relevant today is the idea that dogs don’t necessarily need yearly boosters of some core vaccines. 

They found while reviewing a number of studies that many dogs that received their full primary course of vaccines as a puppy maintain strong immunity for many years, even if they do not receive boosters. 

However, this only applied to a certain kind of vaccine, other kinds, like the leptospirosis vaccine, require more frequent annual vaccination. This means that the practice of giving triennial (every three year) boosters of DHP may result in unnecessary over-vaccination.

Instead of the triennial DHP for all dogs, the WSAVA recommends regular serological testing to monitor an individual dog’s level of immunity

This involves taking a small blood sample from a dog and using a test kit. A positive result for each disease indicates that the dog has immunity and does not need to be vaccinated. A negative result may indicate the dog’s immunity has waned and needs a booster (though a negative result can occur even if the dog has strong immunity). Dogs under 10 can be tested every three years, while older dogs (who’s immunity may be reduced) should be checked yearly.

This means that in the UK, a new puppy would receive a number of primary vaccines (the WSAVA recommends a high number of primary doses to ensure the puppy is fully protected, but that is a debate for another day), then one vaccine at 26 or 52 weeks (again, 26 weeks isn’t common in the UK, so this may be a topic for later). After this, the DHP vaccines should be given no more than every three years, and ideally based on the serological testing results. The L vaccine is still given annually as normal. So for some dogs, they may only ever have the DHP vaccine as a puppy and once at a year old.

This approach is certainly interesting and holds some merit, so it is worth discussing the pros and cons.

Advantages

The major medical advantage of reducing the frequency of vaccination is that it reduces the risk of adverse events

All medicines, regardless of what they do, come with some side effects. This includes vaccines, as those recently vaccinated may know. Vaccines do go through strict testing protocols before being released for general use – despite this, side effects do occur. In dogs, mild side effects are the most common; such as pain, inflammation at the site of injection, mild fevers and sleepiness. More serious side effects are possible, often linked to underlying allergies. Despite this, one US study showed that there were 38.2 adverse reactions in 10,000 dogs. This is around 0.00382%. Rates vary vaccine to vaccine, but are always incredibly low. Regardless of the low number, it makes sense that the less vaccines we give, the less risk there is of adverse reactions. 

Another benefit to the above scheme is that it means vets can only vaccinate what is needed

DHP commonly comes as a single vaccine dose, but you can normally vaccinate against each disease individually. Using serological testing, a dog that showed protection against distemper and hepatitis but was vulnerable to parvovirus could have just the parvovirus (and regular lepto) vaccines that year, again reducing the risks associated with over-vaccination and ensuring that it is fully protected. The WSAVA found that there is no ‘better’ immunity from boosting regularly: “a dog receiving a… vaccine every 3 years will be equally well protected compared with one receiving the same vaccine annually,” so over-vaccination really carries no benefits.

The third major benefit is the savings reduced vaccination produces, both in money and time for pet owners and veterinary practices

Vaccinations certainly aren’t the most expensive aspect of veterinary medicine, and many practices subsidise or offer discounts on vaccines to encourage regular uptake. However, everyone would surely appreciate the saving here and there. On top of this, vaccine appointments take up a lot of time on a vet’s schedule. While vaccination is important, these appointments could be used instead for sick animals or emergencies – although remember that in the UK the lepto vaccine is required yearly anyway, so dogs will still need to be seen once a year.

Disadvantages

The biggest disadvantage to replacing regular boosters with regular serological testing is the cost and availability of these kits

Many practices do not stock them regularly, and they are not cheap. Individual costs will vary from practice to practice, but the WSAVA admits that booster vaccines are almost always cheaper than testing for immunity. This will certainly put a lot of people off. The WSAVA says this is the argument between proper evidence-based veterinary medicine vs reducing costs and increasing compliance. As vets, we often have to balance the gold standard of treatment with individual circumstances. Spending hundreds on a test to avoid over-vaccination simply isn’t practical for many pet owners, even if it is best practice.

The second is the increased risk of disease

The diseases we vaccinate dogs against are quite nasty, sometimes even fatal. As stated above, most dogs will have lifelong protection after their puppy vaccines, but ‘most’ is not all – and it’s a significant percentage (around 40% of dogs unprotected against some disease after 5 years in some studies). Blanket boosters carry the advantage that we know for certain a dog is protected – granted, regular serological testing will do this too, but as stated above this is not always an option. While blanket boosters will result in over-vaccination and increased adverse reactions, it will also avoid under-vaccination and the incidence of disease.

Do Older Dogs Still Need Regular Boosters?

According to numerous studies, for the distemper, infectious hepatitis and parvovirus vaccines, probably not! Immunity appears to be almost lifelong for all these vaccines. However, leptospirosis, as well as non-core vaccines like rabies or Lyme disease do need more regular booster vaccines. So a dog will still need to be seen by a vet at least once a year.

As discussed above, the ideal protocol for an adult dog would be annual lepto vaccines, and serological testing for DHP every 1-3 years, with the DHP vaccines only given if there is a negative test result. And certainly in future more practices may start to implement this policy. Again, this is likely the ‘best’ practice according to evidence-based veterinary medicine. Yet it is certainly not practical for all vets and all owners. The WSAVA admits that “it is simply not possible to produce a set of guidelines that applies equally to each of the 80 WSAVA member nations as there are vast differences between countries…”.

In future, serological testing is likely to get cheaper and more readily available, making the gold standard approach to dog vaccines more affordable for the vast majority. In the meantime, there are lessons that can be learned from the WSAVA, resulting in a better, but maybe not the best, protocol: annual lepto vaccines, with DHP given every three years at the most. As discussed above, this may leave some dogs vulnerable but is overall the best of both worlds for vaccination protocols.

Veterinary medicine is a complex and ever-changing industry. There is often more than one answer, and what we thought was correct may later be proven incorrect. We hope this investigation into dog booster vaccines has shown this!

Further Reading

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