Karen Taylor asked:
How often should our dogs be re-vaccinated (boosters)?
Answer:
Hi Karen, thanks for your question about booster vaccinations. This is an area that’s become quite controversial in the last few years, and there’s a lot of confusion about the subject. In addition, there’s a lot of very poor-quality information out there, so I’ll try to make this quite clear and obvious!
To put it as simply as possible – see your vet every year for a health check, and discuss your vaccination strategy with them.
For more detail… now read on!
What are vaccinations?
Put simply, a vaccination is a way of teaching your dog’s immune system how to recognise and defeat the micro-organism that causes an infectious disease, without the risks (of illness, potential long term health problems or death) inherent in a “natural” infection.
This is achieved in one of three ways:
1) A weakened form of the disease-causing organism.
These are called “modified live” or “attenuated” vaccines, e.g. for Distemper and Parvovirus; the organism included is unable to multiply and/or cause clinical disease, but it is active enough to stimulate a strong immune response. Most modified live vaccines give a stronger and more long-lasting immune response than an inactivated vaccine; however, they aren’t suitable for every disease (because some organisms cannot be weakened enough to make them safe).
2) An inactivated (“killed” or “dead”) form of the organism.
These cannot ever cause disease, but allow the immune system to recognise the protein coat of the organism and therefore attack it next time. They may be used for particularly dangerous or unpredictable diseases such as Rabies or Leptospirosis, but don’t always give such long-lasting protection.
3) Subunit vaccines, introducing part of the organism to teach the immune system what it “looks like”.
For these, part of the protein coat of the target organism is replicated in a lab, and included in the vaccine; this means the immune response is really tightly targeted at one particular, vital, part of the organism. These are used, for example, in the Leishmania vaccine.
There are 2 groups of vaccines – core and non-core. Core vaccines are those that should be given to every dog – they protect your dog and everyone else’s against dangerous, highly contagious and potentially fatal diseases. Non-core vaccines are those that are given to protect dogs that are particularly at risk of a specific condition because of their location, lifestyle, etc.
The core vaccines that every dog should have are against:
- Distemper.
- Parvovirus.
- Canine Infectious Hepatitis.
The vaccine against Leptospirosis is technically non-core; however, it is generally agreed that every dog in the UK is at risk of Lepto (which is spread by rat urine), and so it is treated as a core vaccine by most vets.
The non-core (optional) vaccines available are:
- Rabies (only necessary for pets travelling abroad).
- Parainfluenza (one of the causes of kennel cough).
- Kennel Cough (the bacterial sort, Bordetella bronchiseptica).
- Lyme Disease (only necessary for dogs at high risk, e.g. gundogs, in high risk areas, e.g. the South West peninsula).
- Leishmania (only necessary for dogs travelling to southern Europe).
- Canine Coronavirus (only usually needed in breeding kennels).
If vaccines are so good, why do they need boosting?
Because nothing lasts forever! Eventually, the immune system starts to “forget” how to handle a particular disease organism. Booster vaccines effectively remind the system and refresh the immunity.
However, immunity to different diseases (and different types of vaccine, for that matter) lasts a variable amount of time, and that’s the problem. Some dogs will retain immunity for longer than others – unfortunately, there’s no easy way to tell which dogs are immune to what for how long.
Is there any way to tell whether my dog actually needs a booster?
Not really! The trouble is that although some vaccines operate by producing protective antibodies (e.g. Rabies), others rely on inducing a Cell Mediated Immune Response (immunity that doesn’t rely on antibodies in the blood, but circulating immune cells, e.g. T-lymphocytes and Natural Killer (NK) cells) – such as the Leishmania vaccine, which may not produce any antibodies at all. And most of them probably rely to some extent on both systems.
It’s easy to test the dog’s blood for antibodies (and there are some commercial companies that will do this and say “yes, high levels of antibody, so the dog is protected” or “no, not enough antibody, the dog needs to be vaccinated again”. However, this is not generally considered reliable, because:
- The serological titre (level of antibodies in the blood) can only tell you how much antibody there is in the bloodstream at the specific time the test is done – it cannot tell you whether the levels will remain high for the following 12 months.
- The link between antibody levels and protection isn’t consistent – some dogs utilise other parts of the immune system (cell mediated immunity) – for example, dogs can be protected against Leptospirosis in the presence or absence of significant circulating antibody levels.
So how long does immunity actually last?
How long the vaccine lasts depends on the exact formulation of the vaccine; at the time of writing, the three Core vaccines generally need boosting 1 year after the initial course, then every 3 years. Most Rabies vaccines needs boosting only every 3 years; and the others usually require annual boosters.
To get a license for a vaccine, the manufacturer has to demonstrate that the product has a protective effect, however that is defined. For Core vaccines, they have to demonstrate onset and duration of immunity such as to fulfil the license claim to:
- “Prevent mortality and clinical signs caused by canine distemper virus infection”.
- “Reduce clinical signs of infectious hepatitis and viral excretion due to canine adenovirus type 1 infection”.
- “Prevent mortality, clinical signs and viral excretion following canine parvovirus infection”.
If this cannot be demonstrated to the regulator (in the UK, the Veterinary Medicines Directorate – VMD), they won’t get a license for the product. This means that manufacturer’s recommendations for duration of immunity are those that will protect the vast majority of dogs for the quoted time (3 years or 12 months, depending on the vaccine).
To make life a little more complex, any vet who uses a different vaccination interval, unless they can document a good clinical justification, is technically acting illegally by using the vaccine off-license (i.e. not as licensed by the manufacturer). This sort of behaviour tends to lead to unpleasant interviews with the VMD and has led to vets being struck off (although not, to my knowledge, for vaccine infringements as yet).
Can over-vaccination harm my dog?
There’s no reliable evidence that it can. In cats, every subcutaneous injection (of anything, even saline!) slightly increases the risk of an Injection Site Sarcoma, but despite a lot of scientists, vets and owners trying to find a link, there’s no evidence that it causes any problems in dogs.
That said, absence of evidence is not necessarily evidence of absence, so a responsible approach would be to vaccinate as infrequently as the current evidence suggests is sufficient to provide protection – in other words:
1) Get a health check for your dog at the vets every year.
2) Follow the manufacturer’s recommendations (unless your vet has a particular clinical reason not to):
- Distemper, Parvo and Infectious Hepatitis – boosters every 3 years.
- Lepto – annual booster.
- Rabies – boost every 3 years.
- Other Non-core vaccines – usually every year.
I hope that helps; this is a really controversial area in some quarters, but the evidence base for the current vaccination protocols is pretty secure, and it is what I’d advise you to follow.
David Harris BVSc MRCVS
