FCV infection is ubiquitous and may induce severe disease. ABCD therefore recommends
that all healthy cats should be vaccinated against FCV. Although vaccination provides good
protection against acute oral and upper respiratory tract disease in most cases, it does not
prevent cats from becoming infected and from shedding FCV afterwards (Radford et al.,2006). In addition, there is currently no vaccine available in Europe that protects against all
FCV field strains.
Currently, FCV is combined with FHV-1 in divalent vaccines (only in some countries) or,
more commonly, with additional other antigens. Both modified live and inactivated parenteral
vaccines are available. Modified live intranasal vaccines are no longer available in Europe,
but still current in the USA.
FCV vaccines provide protection mainly by inducing humoral immunity (VN antibodies). As
the virus can mutate quickly, field strains could evolve resistance to any vaccine-induced
immune response, particularly if a vaccine is used for a prolonged period of time in the
population (Lauritzen et al., 1997). Though there is little published evidence available,
ongoing field studies support this hypothesis. They are conducted to obtain more information
about the strains circulating in Europe, and vaccine companies are seeking to identify newer
strains that provide wider cross protection (Poulet et al., 2005). The most commonly used
vaccine strains of FCV are: F9, which is the oldest, isolated in the 1950s, FCV 255, and two
new strains G1 and 431 (Poulet et al., 2000; Poulet et al., 2005). Some vaccine companies do
not state the strain of virus used in their vaccine.
In the absence of compelling published data, it is difficult to make a general recommendation
about which vaccine strain to use. However, if disease is occurring in fully vaccinated cats
that are housed in groups, then changing to a different vaccine antigen may offer advantages.
The impact of vaccination on the shedding of field viruses is controversial, with one study
showing a moderate reduction (Poulet et al., 2005) whilst others show that vaccination might
actually extend the period of virus shedding post infection. (Dawson et al., 1991; Pedersen &
Hawkins, 1995). Live parenteral FCV vaccine strains can be shed, although it seems rare.
(Pedersen & Hawkins, 1995; Radford et al., 1997, 2000, 2001; Coyne et al., 2007).
Live vaccines retain some pathogenic potential and may induce disease if administered
incorrectly, e.g. when accidentally aerosolised or spilled on the skin and ingested (Dawson et
al., 1993; Pedersen & Hawkins, 1995; Radford et al., 1997; Radford et al., 2000). However,
this appears to be a rare event.
Cats that have recovered from caliciviral disease are probably not protected for life against
further episodes of disease, particularly those caused by different strains. Therefore,
vaccination of recovered, healthy cats is generally recommended, even in situations where
FCV is endemic.
The value of serological tests in predicting protection is limited, because antibodies to the
calicivirus strain used in a laboratory test may not necessarily protect against the strains that
the cat will subsequently be exposed to in the field.
ABCD recommends that all kittens should be vaccinated against FCV. Because MDA can interfere with the response to vaccination, the primary course of vaccination is usually started at around nine weeks of age, although some vaccines are licensed for use at an earlier age. Kittens should receive a second vaccination two to four weeks later, but not earlier than at twelve weeks of age. This protocol has been developed to ensure optimal protection. However, due to a longer persistence of MDA some kittens may fail to respond to this protocol (Dawson et al. 2001). Therefore, in high-risk situations, particularly where FCV has been shown to cause disease in vaccinated kittens, a third vaccination at 16 weeks should be considered. We recommend using the same brand for the entire primary vaccination course. Older cats of uncertain FCV vaccination status should also receive two injections with an interval of two to four weeks, using vaccines containing the same virus strains. This applies even if the vaccine contains modified live virus.
The issue of recommended intervals between boosters is still controversial. However, based
on positive study results published by several independent groups, ABCD recommends that
boosters should be given at triennial intervals to protect individual cats against FCV field
infections. These cats are in low-risk situations, mainly indoor-only cats with little or no
contact to others. However, owners should be made aware that as time since the last
vaccination increases, the degree of protection will decrease. Cats in crowded high-risk
situations (e.g. boarding catteries) should be revaccinated at yearly intervals. For other cats,
an informed decision should be made on the basis of a risk-benefit analysis.
The ABCD recommends a single injection if the interval since the last vaccination is less than
three years. If the interval exceeds three years, two vaccinations would ensure optimal
protection. Boosters using FCV vaccines from different manufacturers are acceptable.
The ABCD appreciates that single-component FCV vaccines are currently unavailable.
Annual boosters that protect against other antigens may in practice entail more frequent
boosters than triennially.