Classification of feline vaccines
As with canine vaccines, feline products are now categorized as core, noncore, and not generally recommended. As the names
imply, vaccines that all cats should receive are core, while noncore are optional based on lifestyle, local disease prevalence,
and risk/benefit ratios. These categories are from the 2006 AAFP guidelines (see Proceedings on core vaccines).
Core vaccines include panleukopenia, herpes, calici, and rabies. Vaccines classified as noncore and not generally recommended
are discussed below.
Leukemia virus (FeLV)
There is ongoing debate as to which cats should be vaccinated for FeLV, and if so when and how often. It can be difficult
to assess an individual cat's risk of being exposed. Certainly cats that spend time outdoors are more likely to be infected
than indoor cats, but lifestyles and confinement can change through a cat's lifetime. Owners may believe they will keep new
them indoors for life, but cats may have other ideas.
Kittens are much more susceptible to FeLV infection than adults. Cats older than 1 year of age can be exposed but are less
likely to become persistently infected. The minimum age that kittens can be vaccinated (according to label recommendations)
is 8, 9, or 10 weeks, depending on the brand, so FeLV vaccine is typically not given to kittens presented at 6-8 weeks of
After two vaccinations 3-4 weeks apart (for either kittens or adults), a booster is given 1 year later. Afterward, annual
revaccination is recommended as extended DOI studies have not been performed. A 1-year challenge study is available for one
vaccine (Leukocell 2, Pfizer). Kittens were vaccinated at approximately 9 and 12 weeks of age and challenged oronasally one
year later. 14/18 vaccinates (78%) were protected. However, only 9/15 (60%) of controls (nonvaccinates) became infected, so
the preventable fraction was 63%. The likely explanation for the 6/15 young adult cats that were not infected by the challenge
is age-related resistance. The 4/18 cats not protected despite vaccination indicates that FeLV does not always induce immunity.
A relative lack of efficacy was observed in an outdoor colony where 3 of 5 FIV+ cats became infected with FeLV even after
annual vaccination (Leukocell 2). All 19 FIV- cats were protected from FeLV up to 5 years with annual vaccination.
Efficacy studies of FeLV vaccines are available but difficult to compare, as different models of vaccination, challenges,
and outcomes were used. An older study of one product showed that immunity lasted for 3 years, and based on knowledge of immune
system memory, it is reasonable to expect that annual revaccination is not absolutely necessary. Another study found that
cats vaccinated and then challenged with FeLV did not have viremia (ELISA negative) but still tested positive for proviral
RNA by quantitative and real-time RT-PCR testing. Therefore, FeLV vaccination is unlikely to provide "sterilizing" immunity
but does seem to protect against viremia, shedding, and clinical disease.
The safety of FeLV vaccines has been a concern ever since the recognition of injection-site (or vaccine-associated) sarcomas.
Some studies suggest that both rabies vaccine (RV) and FeLV are associated with an increased risk of post-vaccination chronic
inflammation, most likely due to adjuvants. Some practitioners now limit FeLV vaccines to cats at risk of exposure, while
others recommend routine vaccination to most or all cats. The current AAFP recommendation is to vaccinate all kittens due
to their higher risk of infection and unknown indoor/outdoor status later in life. For adult cats, more selective use is appropriate,
considering that inflammation and neoplasia (while rare) can be a significant problem.
Another strategy to minimize the risk of FeLV vaccine-associated sarcomas is to use a nonadjuvanted product. Currently, only
one type is available in the U.S. that is produced with recombinant technology and delivered transdermally with a needle-free
injection device. Studies and clinical experience to date suggest that post-vaccination inflammation does not occur, which
indicates that neoplastic transformation is unlikely. This vaccine stimulates both humoral and cell-mediated immunity because
of delivery directly to dendritic (antigen-presenting cells) in the skin. This is in contrast with other vaccines that are
injected into the subcutaneous space and are not in direct contact with the immune system.