Vaccines (as we are all too aware at the moment!), are life-saving preventative medicines. Worldwide, they reduce both the frequency and severity of many infectious diseases in humans and animals. Whilst rigorously tested in comprehensive safety trials before approval for use and licensing, there exists annually, a very small incidence of adverse reactions. Typically, the frequency of such reactions is tiny and only remains a fraction of just one percent. 

Unfortunately, one rarely documented problem, can be a feline injection site sarcoma (also known as FISS). This article will now discuss this unique, species specific disease.

What is FISS?

In cats, an invasive form of cancer, the feline injection site-sarcoma (FISS) is the most serious potential adverse effect following vaccination. This unique form of skin cancer is an aggressive, locally infiltrative, malignant tumour. It usually develops at the site of a previous injection. Tumour growth is typically both rapid and intrusive into the local surrounding tissues. In up to a quarter of cases, metastatic spread (distant spread of the tumour), may occur to the lungs. 

What is the incidence?

An article published in the veterinary literature in 2019, cited the estimated incidence of FISS in registered cats in the UK, as 1 in 16,000 – 50,000. The disease appears to be more commonly diagnosed in certain countries other than the UK. The USA for example, reports a higher incidence of disease. America has even established a task force to help Vets understand, manage and prevent these tumours from occurring. 

A link with specific types of vaccines “adjuvanted vaccines,” was initially noted in 1991 in the USA. An adjuvanted vaccine is one in which a killed (inactivated) vaccine has a carrier molecule linked to it. This carrier molecule enhances the local inflammatory response, which is vital to enhancing the necessary immune response. At this time in America, there appeared an increase in incidence of FISS tumours associated with the increased administration of (adjuvanted) rabies and FeLV (feline leukaemia virus) vaccination. The more intense local inflammation stimulated by these vaccines, was thought to act as a trigger for malignant cancerous change in cells.

Which cats are predisposed?

Middle aged cats seem predisposed to FISS’s albeit they can develop in any age of cat. The most likely process for development of these tumours suggests that induced long-term fibrous inflammation at the injection site, acts as a trigger for subsequent malignant cancerous change of cells. Despite extensive research however, this remains a theory as no definitive proof of the development of FISS has been discovered. 

Clearly not all cats who receive a vaccination develop FISS’s though. There may also be a genetic contribution associated with an increased risk of developing a FISS, or yet unidentified factors involved. 

So, are vaccinations the only possible cause?

Not solely. Whilst vaccinations have been implicated with FISS’s, many other substances placed under the skin, can also play a role. Long-acting steroid injections, antibiotics, suture material and in some very rare circumstances, even microchips have played a role in the cause and development of the tumour. Repeated trauma (in the form of repeated injections) within the same site, is also thought to play a significant role.


As UK vets, we only vaccinate against the core infectious diseases that are relevant to any one individual cat within our country. Given we remain a rabies free country for example, the rabies vaccination (against UK residing, non-travelling cats) is not routinely undertaken. The mantra is to vaccinate MORE individual cats (thus improving overall feline “herd immunity”), but to vaccinate each individual cat as INFREQUENTLY as possible. 

The lifestyle of your cat will also enormously influence their need for protective cover from vaccination. 

For example, a singly housed, exclusively indoor cat who is not exposed to, or never meets other cats, does not need protecting against feline leukaemia virus FeLV (since close contact with another cat is the only potential viable route of transmission). Outdoor cats however, should have FeLV cover since they may have contact and exchange blood or saliva with other cats, through fighting and close contact.

Vets also follow WSAVA guidelines and reduce vaccination frequency to triennial (every 3 years) where advised. This is currently possible for most UK licensed feline infectious enteritis and feline leukaemia virus vaccines. Non adjuvanted vaccines are clearly preferred and used where possible.

Additionally, further measures can be taken to reduce the frequency of development of FISS. 

Vaccines (and other injections), should potentially be administered in sites more amenable to aggressive surgical removal (and thus a cure), should this be required (if a FISS were, sadly, to develop). This has been taken to quite an extreme in the USA, where most vaccinations are now given into either the lower half of a cat’s leg or the lower half of the cat’s tail. Whilst sad to have to think about such worrying side effects from a vaccination, the use of such locations would enable amputation of the limb or tail, and thus a probable cure from cancer.

Research has shown that warming the vaccination prior to administration has been associated with a significantly reduced risk of tumour occurrence. Allowing a few minutes for the vaccination to come to room temperature before administration is beneficial and advised. 

In the UK, given we see less FISS’s compared to other countries, some vets will use the lateral abdominal body (the side of your cat) as a site in which to place an annual vaccination. 

This is a site from which, aggressive surgical removal of a FISS could be better achieved if required (rather than in between the shoulders, where the shoulder blades and spine would limit radical surgery). Furthermore, some UK clinics may rotate the location of annual vaccination, on a year-by-year basis. This serves to counteract the link with repetitive trauma in one location, being a risk factor for the disease. Recording in the clinical notes precisely where a vaccine has been given, is also a useful practical step.

So, what if I find a lump after my cat has been vaccinated?

First of all, don’t panic. It is not uncommon for a small inflammatory (not cancerous) lump to develop at the site of a previous vaccination. In the vast majority of cases, that lump will grow only to a certain, finitely small size, before reducing again over a 1-3week period.

Any presence of such a “lump” should however, be recorded, measured and closely monitored by your Vet. Do ask to be seen and make sure the size if the lump is accurately documented. This “post vaccinal monitoring” allows early detection of sarcomas, which is essentially critical to ensure that they can be removed successfully early, should such a rare event happen to your cat.

You may also be interested in;