After several experimental vaccines had been described [Jarrett et al., 1975; Jarrett et al.,
1974; Pedersen et al., 1979], the first FeLV vaccine in the field was introduced in the USA in1984. This vaccine was based on conventionally prepared FeLV antigens, and it protected
cats from FeLV viraemia [Lewis et al., 1981].
A number of FeLV vaccines are now available in Europe. Some of these are based on new
developments in recombinant DNA technology. One such vaccine consists of the viral
envelope glycoprotein as well as part of the transmembrane protein expressed in E. coli
[Kensil et al., 1991], and this was the first genetically engineered small animal vaccine. The
most recent FeLV vaccine uses a canarypox virus vector that carries the genes for the
envelope glycoprotein and the capsid protein [Tartaglia et al., 1993]. There is a single round
of replication by the vector virus following vaccination, resulting in the expression of the
inserted FeLV genes. In contrast to other cat vaccines, neutralising antibodies do not develop
following vaccination with this product. The protective effect is achieved by stimulating
cellular immunity which leads to rapid development of neutralising antibodies if vaccinated
cats encounter field virus [Hofmann-Lehmann et al., 2006; Lehmann et al, 1991].
The differences between the various brands of FeLV vaccines are considered to be more
significant than those for other feline infectious diseases, and there is evidence that this is
reflected in differences in performance, particularly related to efficacy of protection.
Comparison of the results of vaccine efficacy studies can be misleading because of
differences in the protocols used – such as the route of challenge, the challenge strain used
and the criteria for defining protection [Sparkes, 2003]. Different studies on the same vaccine
have sometimes led to contrasting results. The first FeLV vaccine and some other vaccines,
which have now been withdrawn from the market, have performed very badly in some
independent vaccine efficacy studies indicating poor protection.
The European Pharmacopoeia defines certain criteria for assessing the efficacy of protection
achieved by FeLV vaccines. This takes into account the difficulty in infecting some healthy
control cats with a single experimental challenge and the criteria include a minimum
acceptable infectivity rate in controls to confirm that an acceptably strong challenge has been
provided. The “natural resistance” of some of the control cats is taken into account in
calculating the level of protection achieved by vaccination and this is expressed as the
preventable fraction [Scarlett & Pollock, 1991].
Some protocols for studies for assessing vaccine efficacy have been developed based on a
“natural” challenge of FeLV – by co-mingling viraemic “challenge” cats with trial cats.
Although these protocols are not in agreement with the European Pharmacopoeia, they take
account of the natural mode of transmission of FeLV which is generally based not on a singlelarge exposure but chronic exposure over a period of time, usually through cohabiting of
infected viraemic cats with susceptible cats, and clinical experts regard this as providing a
more realistic indication of the efficacy of protection vaccines are likely to provide in the
field.
No FeLV vaccine is likely to provide 100 % efficacy of protection and none prevent infection
[Hofmann-Lehmann et al., 2007]. Recent studies have demonstrated that cats that are able to
overcome p27 antigenaemia without exception become provirus positive in the blood and also
positive for viral RNA in plasma, although at very low levels compared with persistently
viraemic cats [Hofmann-Lehmann et al., 2007]. These experiments confirm that FeLV
vaccination neither induces sterilising immunity nor protects from infection.
Long term observation of vaccinated cats following experimental challenge indicates that low
level RNA viraemia and persistence of low levels of proviral DNA can be considered as not
clinically significant and these cats can be regarded as “protected”.
In most circumstances FeLV should be included in the routine vaccination programme for pet
cats. It provides good protection against a potentially life-threatening infection and the benefit
for most cats considerably outweigh any small risk of serious adverse effects. In situations
where the possibility of future exposure to FeLV can be discounted, vaccination is not
required. Geographical variations of the prevalence of FeLV may influence the decision
whether or not to vaccinate against FeLV. In some European countries FeLV has largely been
eradicated and there may be important local variations in the prevalence of FeLV within
countries where the virus is still a significant health issue that may be taken into account. The
circumstances of individual cats may also be a factor and if it can be assured that a cat will
not be exposed to FeLV, vaccination is unnecessary.
However, owners’ circumstances may change which may influence their cats’ lifestyle and
lead to potential exposure in a cat that was previously at no risk of encountering FeLV,
particularly when moving house. This possibility should be considered especially in kittens
presented for primary vaccination.
Vaccination should be carried out in all cats that have a potential risk of exposure. In such
cases it is recommended that kittens be vaccinated at the age of 8 or 9 weeks and 12 weeks
together with core vaccinations. [Brunner et al., 2006]. As the combination of differentimmunogens within one syringe is only legal when the company has registered it for the
country of interest; the local veterinary regulations should be carefully consulted.
If the FeLV status is unknown, any cat should be tested for presence of FeLV antigenaemia
prior to vaccination in order to avoid "vaccine failures", which are likely when cats infected
prior to vaccination develop FeLV-related clinical signs. If FeLV infection prior to
vaccination is unlikely, testing may not be needed (e.g. kittens from a FeLV negative mother
and father which had no contact with other cats).
No data have been published to support a duration of immunity (DOI) longer than 1 year after primo-vaccination. Therefore, most vaccine producers recommend annual boosters. However, considering the significant lower susceptibility of adult cats to FeLV infection, the ABCD suggests that in cats older than 3-4 years, a booster every two to three years would be sufficient.