This is always a controversial subject… but sadly there’s a lot of misinformation out there. As a result, it can be very hard to find accurate data in an easy to read form. In this blog I’m hoping to take a brief spin through vaccines for pets. Specifically, the thorny issues of what vaccines to give, and how often they need to be repeated. Because, like most things involving real living creatures, there aren’t any simple answers! If anyone claims there is a simple rule that answers these questions (e.g. “don’t ever vaccinate an animal”, or “vaccinate every animal for everything every year”), the odds are that either they aren’t aware of the science behind modern vaccines, or they’re trying to sell you something.


What is a vaccine?

Essentially, a vaccine is a way of harnessing the animal’s natural defences and supercharging them. Therefore, they respond more powerfully to infection, by working with nature rather than against it.

Does that sound like an odd way to phrase it? It’s absolutely true though. The immune system is ridiculously complex. It has a myriad of different cells and circulating proteins, all communicating with each other (and, we now believe, with the gut and the central nervous system) to defend the animal against invaders.


That said, while the mammalian immune system is indeed a ruthless killing machine, it is not perfect. It takes time (days to weeks) for the immune system to work out how to combat that disease. However, in that time, the infection is running rampant through their system… and sadly, a percentage of infected animals will die of disease. In dogs infected with Canine Distemper, for example, the mortality rate may even exceed 50%.


So how does a vaccine actually work?

A vaccine is a method of “showing” the immune system the disease causing agent, while minimising the risk that the full-blown disease will follow. We often use a weakened (or “attenuated”) form of the infectious microbe – it can multiply in the body, but only very slowly, allowing the immune system to defeat it with ease. These “Modified Live Vaccines” (MLVs) usually give the most effective, longest lasting protection, because the immune system is essentially doing what it does best – locating and destroying a real threat. We know the actual risk is minimal, but the white blood cells (B and T lymphocytes) do not!

The second approach is to use a killed version of the organism, that can never cause disease. This is superficially attractive, as it should reduce to zero the (already very low) risk that the vaccine strain will cause disease. However, the immune system also isn’t stupid – it generally knows that dead bugs aren’t a threat, and largely ignores them. As a result, the vaccine contains an extra compound, an adjuvant, to cause local tissue irritation. This persuades the immune system that while the bug LOOKS dead, it’s worth attacking it anyway. It is these adjuvants that are at the centre of most concern about vaccination, although the evidence that they are responsible for widespread disease in dogs or cats is still lacking.



What do we vaccinate against?

In the UK, we routinely vaccinate dogs against Canine Distemper Virus (CDV), Canine Parvovirus (CPV) and Canine Adenovirus 2 (CAV-2, that causes infectious hepatitis). These are the “core vaccines” as defined by the World Small Animal Veterinary Association, WSAVA. However, in the UK, Leptospirosis is considered to be a high risk, and given the threat that an infected dog poses to human health, most vets recommend vaccination against leptospirosis as well. Vaccines are also available for 2 kennel-cough causing organisms (Canine Parainfluenza, or CPi, and Bordetella bronchiseptica), for rabies (only for travel abroad as “classical”, i.e. dog-based, rabies is not currently present in the UK), for Lyme Disease, Canine Herpesvirus (although the vaccine can be hard to get hold of) and Leishmania.

In cats, we routinely vaccinate against Feline Panleukopenia (FPV) and 2 cat-flu organisms, Feline Herpesvirus (FHV) and Feline Calicivirus (FCV) – again, these are termed core vaccines. Additionally, we can vaccinate against Chlamydophila (Feline Chlamydia, a cause of cat-flu) and Feline Leukaemia Virus (FeLV).


Are vaccines safe?

In general, yes, they do appear to be relatively safe. The incidence of significant side effects is in the region of 1 in 10,000 doses. For all the hysteria about “Vaccine X kills dogs!!!” or whatever you read online, it’s worth bearing in mind that the vast majority of dogs and cats are vaccinated without any ill effects. For most dogs and cats, most of the time, the risk of a vaccine side effect is much lower than the risk of the infectious disease.

Are you saying they’re perfectly safe?

Of course not. In fact, we shouldn’t expect them to be – any medication capable of changing the function of a major body system is in and of itself capable of causing harm in some situations. Sadly, any drug claiming to be “perfectly safe” is either being sold by a liar, or isn’t doing anything at all.

So what can go wrong?

The biggest risk from a vaccine is over-activation of the immune system – resulting in local swelling and discomfort before the lymphocytes etc realise that they’ve made a mistake and shamefacedly slink back to their lymph nodes.

More rarely, they may massively overreact, leading to anaphylactic reactions – although these seem to be much rarer in dogs and cats than in humans, for example. However, in a dog or a cat that is genetically programmed to develop autoimmune disease, it is theoretically possible that vaccination will trigger a latent condition. This seems to be quite rare, but is biologically plausible.

Can’t they cause cancer in cats?

Yes – in cats, we have an additional risk, that of Feline Injection Site Sarcoma (FISS). It is now well established that the act of injecting anything into a cat is capable of triggering a tumour. These masses are malignant and locally invasive, and often hard to remove unless caught early. However, they are also rare in the UK – while in the US they seem to occur in about 1 in 10,000, in the UK it seems to be rarer (perhaps 1 in 20,000 injections).

Compared to the risk of panleukopenia, cat flu, or feline leukaemia, your cat is very unlikely to suffer a problem… But sadly, that doesn’t mean it won’t or can’t happen. There is a lot of research going on, and many vets suspect that vaccines without adjuvents might be safer. However, we don’t really know for sure. The development of FISS is so rare that it’s difficult to study!

Why do vaccines need boosting?

Because the immunity posed by a vaccine is not necessarily lifelong. Some animals may mount a lifelong response to some organisms, but others (given the same vaccine) may not. The usual programme of vaccines involves a puppy- or kitten course, and then a booster at 12 months old. The reason we have the course is twofold.


The modified live vaccines (CDV, CPV, CAD and FPV) will give long-term immunity after only a single dose… IF (and only if) the immune system responds to it fully. Puppies and kittens are protected from infectious disease by antibodies they get from their mothers. These wear off somewhere between 8 and 14 weeks – but the “waning” of this maternal protection is very variable. So, we give 2 doses, in the hope that we will “catch” most animals after their mother’s protection is gone, but before they catch the real disease.

Many other vaccines (Leptospirosis in particular) need 2 doses to trigger immunity – because the way they work is rather different.


So why do we need an annual booster?

The annual booster is there to “mop up” any animals that didn’t respond originally, although 16 weeks  would be better. However, many animal owners aren’t willing to keep coming back, so most practices have compromised on a 12 month one.



How long do the vaccines keep working for?

This is of course the million pound question!

Non-core vaccines

For these, loss of immunity over time is usually fairly predictable. There is very good evidence that Lepto vaccine immunity wanes much, much faster, (under a year). The cat flu vaccines give strong protection for a few months, which then starts to gradually fade away over time. There is still some protection at 3 years, but the WSAVA does advise high-risk cats are vaccinated annually.

Core vaccines

For the core vaccines, though, the situation is very different. You may well have read that dogs develop “lifelong immunity” after a single dose of parvo or distemper vaccine. This claim is largely based on the work of a researcher called Shultz in 2006, whose analysis suggested that the minimum duration of immunity for many of these diseases was 9 years by serology (i.e. they still had high levels of antibodies for 9 years after vaccination). That said, proven immunity by challenge (whether the animal was actually protected from infection) was significantly lower. His results suggested that it worked out at 7 vs 9 years for CAV, 5 vs 9 years for CDV and 7 vs 9 years for CPV, although both showed as as 7.5 years for FPV.

So isn’t that quite straightforward?

However, we now know that (surprise surprise!) life isn’t that simple. The rate at which the vaccine immunity declines seems to be very, very variable between individual animals. Some, maybe most, dogs may well retain immunity lifelong; however, in others it rapidly fades out over 3 years or so. The vaccine manufacturers therefore recommend a 3 year vaccination interval, because they can prove protection of 98% of vaccinated animals within that window. Some people have rubbished this theory of variable durations of immunity. However, a 2018 study by Vila Nova and others appears to have proven the massive variability in individual animals’ response.



Don’t vets usually boost all the vaccines every year?

No – not for many years. Recent polls of the veterinary profession have found that only a very small minority recommend this. The confusion has come about because a single syringe routinely contains all the components of the vaccine. In the UK, the standard protocol is to give the longer-lasting elements every 3 years. Only the short-lasting ones are given annually. So distemper, parvo and hepatitis for dogs, and panleukopenia for cats every 3, Leptospirosis and usually cat flu annually.


How do I know if my pet needs a vaccine or not?

This is the problem – we want every pet to be protected from these really nasty diseases. I’ve lost patients to Parvo, Feline Leukaemia and even Cat Flu, and I never want to see a preventable case. However, many people, quite reasonably, also don’t want to be vaccinating if they don’t have to. At the moment, the best compromise seems to be titre testing – measuring the levels of antibody in the blood. If they are high enough, then they don’t need vaccinating now, so retest them in another year or so. If they’ve dropped, give them a booster.

There are of course problems with this. For example, it is not possible to titre test against Lepto, or Cat Flu, because antibody levels do not reflect protection. It is also often more expensive to do a titre test than just to vaccinate.

However, I would urge animal owners that if you are concerned about vaccination, don’t just refuse to have it done! That will potentially leave your pet at risk. Instead, titre test, and see whether they actually do need it this year or not!



Do you want to know more?

I’ve included links to most of the main research papers, but the biggest and best source of information about this topic are the WSAVA Vaccination Guidelines. They’re prepared by a board of genuine experts in veterinary immunology and vaccine pharmacology… As it happens, one of whom taught me at vet school. They aren’t paid for by the drug companies, and aren’t out to boost veterinary profits. Their aim is to try and find the optimal solution to protect our pets as well as possible. They also have a range of additional material here.