Are vaccines safe for pets?

The aim of vaccinating is to keep our pets safe. Very rarely, animals can have adverse reactions, but on a balance of potential risks of these adverse events against protection from fatal diseases such as parvovirus or myxomatosis, the veterinary profession strongly advocates vaccination as a safe way to keep our animals healthy.

Let us take a closer look at what exactly vaccines are, the potential risks, and how veterinarians are endeavouring to make vaccines as safe as possible…

Vaccines: what are they?

Vaccines can be modified live vaccines (ML) or killed vaccines.

In ML vaccines, the organism which would cause disease, for example, feline herpes virus 1, is altered in a way in which it can no longer cause disease. However, the organism will still send out an alert to the immune system which will make the cat’s body produce antibodies. It does this because it contains something called an antigen; a substance which is recognised as a potentially dangerous toxin or organism and which simulates an immune response.

For dead vaccines, the organism is dead but still stimulates an immune response, as it is still “wearing” its antigen.

Vaccines are important as they contain the antigen to stimulate an immune response without the risk of causing an infection. Antibodies are the body’s defence against disease; think of them like the guards of your body against viruses and bacteria! By exposing your dog or cat to modified live or dead vaccines, you stimulate their immune system to produce antibodies, without exposing them to a potentially dangerous disease. This means, should your cat then be exposed to the real, live organism, (e.g. FeLV while out on a walk) your cat’s body will recognise the threat and have its guards primed and ready for attack – or the antibodies will be ready to “fight off” the real disease when your dog encounters it!

Vaccines: what are the risks?

Why would owners be worried about giving protection to their pets from vaccines?

The main reasons are either that the vaccine won’t work, or that their pet will suffer from an adverse reaction. Adverse reactions may be classified as “serious” or “non-serious”.

Non-serious reactions would be things like temporary injection site swellings, hypersensitivity reactions, mild upset stomachs and being a little ‘off colour’ for a day or so. Serious adverse reactions include life-threatening clinical signs, significant disability, or even death, though thankfully these sorts of reactions are incredibly rare.

A further concern for some cat owners is “injection site sarcomas”. Feline injection site sarcomas (ISS) are tumours of connective tissue which can be associated with vaccination. These tumours are malignant, meaning they can invade other sites in the body, and severely impact upon your cat’s health. Thankfully, the risk of these is very low, at roughly 1 in 12,500. If they do occur, they can be removed surgically, or by using a combination of radiation and chemotherapy.

So, how do vets ensure that vaccines are as safe as they could be?

Vaccines undergo rigorous testing before they are sold to the public for use in our pets. This involves in vitro testing, i.e. in petri dishes in a lab, and in vivo testing, i.e. trialling the vaccine in laboratory animals.
Reporting adverse reactions. Veterinarians will report any adverse vaccine reactions to the Veterinary Medicines Directorate, should they occur. This allows manufacturers to evaluate their vaccine’s efficacy and safety, to constantly improve their ability to help animals. It adds to the database of safety about that particular vaccine.
Reducing vaccine load. The World Small Animal Veterinary Association (WSAVA) has developed a set of guidelines using evidence-based veterinary medicine to help veterinarians adopt vaccination policies which offer protection to our pets from life-threatening diseases, with the minimum amount of vaccination. Why would we want to vaccinate our pets as little as possible? Firstly, we want to reduce the “vaccine load”; this means that we do not want to over-vaccinate our pets, as this might, in theory, increase the likelihood of adverse reactions (although there’s no evidence that it actually does in practice). Secondly, the veterinary profession does not want to charge clients for unnecessary procedures. For this reason, a policy of “core” and “non-core” vaccines is being developed and constantly re-evaluated.
Core vaccines are those which protect your animal against life-threatening diseases. For dogs, this includes protection from distemper, adenovirus and parvovirus (canine parvovirus type 2). For cats, this is parvovirus (feline parvovirus), calicivirus and herpes (feline herpes virus type 1). For rabbits this includes myxomatosis and viral haemorrhagic disease.
Non-core vaccines are those which are only required by “at risk” animals; the risk to your pet is determined by the prevalence of the disease in your area, or lifestyle. Lifestyle can encompass potential sources of infection such as going into kennels, or cats going outside and encountering other cats.
Vaccinating appropriately. This means vaccinating animals who are healthy, and capable of mounting an immune response to offer them sufficient protection from these deadly viruses.
Animals who are suffering from another disease at the time of vaccination will not be able to mount an appropriate immune response to the vaccine, meaning that they may not have the antibodies to protect them from the disease should they encounter it again. For this reason, your vet will perform a health check to ensure they do not have any coughing, sneezing, diarrhoea, vomiting or general sickness.
Young animals get protection from certain viruses in milk from their mothers; these are termed “maternally derived antibodies”. These antibodies interfere with the vaccines’ abilities to work; for this reason, it is recommended that kittens be vaccinated at 9 weeks, and puppies at 8 weeks. There is great variation between litters, and for this reason, it is recommended that puppies and kittens receive another vaccine, generally at around 10 – 12 weeks, to provide a complete vaccine course.

To summarise – are vaccines safe?

Yes; they are tested extensively before commercial use, and any adverse reactions must be reported to the Veterinary Medicine Directorate.

Yes; vets endeavour only to vaccinate healthy pets who require vaccination, to reduce the chance of any adverse reactions.

No; no treatment is guaranteed not to cause adverse reactions, but fortunately serious reactions are very rare and on a balance of the risk of lethal diseases against the risks of vaccination, vaccination is certainly seen as the safer option.

Please do speak with one of our vets if you are concerned about vaccination. From all of us, we wish you and your pets a happy and healthy life together.