FHV infection is common and may induce severe disease. ABCD therefore recommends that
all cats should be vaccinated against FHV. FHV-vaccines provide protection by inducing both
humoral immunity, associated with a serological response, and cellular immunity.
Vaccination provides good protection against clinical disease, but in common with many
localised respiratory tract infections, it does not provide 100% protection (approx. 90+%
reduction in clinical scores has been achieved following experimental challenge soon after
vaccination)(Gaskell et al., 2007). Less effective protection can be expected in some
vaccinated individuals in particular circumstances following exposure to virus, e.g. extreme
challenge, immunosuppression. There is no evidence that any variation in isolates of FHV
creates problems in protection provided by vaccination.
Vaccination protects from the development of clinical signs, but not necessarily from
infection. However, there is some evidence that it can reduce subsequent excretion of virus.
(Gaskell et al., 2007).
Currently, FHV vaccines are usually combined with FCV, either in divalent vaccines (only in
some countries) or, more commonly with other antigens. Both modified live and inactivated
parenteral vaccines are available. Subunit FHV-vaccines and modified intranasal vaccines
have been available previously or are available outside Europe, however they are not
currently available in Europe.
Both inactivated and modified live FHV vaccines have relative advantages and disadvantages.
There is generally little reason to prefer any specific FHV vaccine for routine vaccination,
particularly since these are all based on a single serotype. Modified live vaccines retain some
pathogenic potential and may rarely induce disease if administered incorrectly, i.e.
accidentally aerosolised or intake of vaccine virus spilt on the skin.
The value of serological tests in predicting protection is controversial. Methodological issues
can complicate comparison of titres and some suggest that titres are not good predictors of
protection. In other studies, cats without any evidence of seroconversion appear to show
protection (Lappin et al., 2002; Mouzin 2004). Cats that have been vaccinated usually develop
an anamnaestic response following exposure.
ABCD recommends that all kittens should be vaccinated against FHV. Maternally derived
immunity can interfere with the response to vaccination and 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, with the
second given around twelve weeks of age. This protocol has been developed to ensure
optimal protection. For longer intervals, no information is available and a new primary
vaccination course should be considered.
In contrast to vaccines against certain other infectious agents, where single vaccination is
acceptable for cats of unknown or uncertain vaccination status, in the case of FHV, they
should also receive two vaccinations at an interval of two to four weeks, irrespective of the
vaccination type.
Vaccination against FHV prevents disease, reduces virus shedding and recrudescence.
Although the issue of recommended intervals between boosters is controversial, in view of
currently available scientific evidence, ABCD recommends that boosters should be given at
annual intervals to protect individual cats against FHV field infections, with the exception of
cats in low-risk situations (e.g. indoor-only cats without contact to other cats). In these cases,
three-yearly intervals would be recommended. An informed decision should be made on the
basis of a risk-benefit analysis, but annual boosters are particularly important to cats that may
be exposed to high risk situations e.g. entry to boarding catteries, breeding cats.
Experimental studies and serological studies in field situations clearly indicate that immunity
against FHV lasts longer than one year in most vaccinated cats (Lappin et al., 2002, Mouzin
et al., 2004). However, there is a significant proportion of cats for which this is not true. Field
studies have shown that almost 100% of cats either have serological titres against FCV and
FPV, or show an anamnestic response following administration of a booster vaccine, but
around 30% of the cat population appear to have no detectable titres against FHV and around
20% fail to show an anamnestic response following booster vaccinations (Lappin et al., 2002,
Mouzin et al., 2004). Assessment of the duration of protection is complicated by failure of
vaccination to provide 100% clinical protection shortly after vaccination has been
administered, but in experimental vaccine efficacy studies, the efficacy of protection afforded
by vaccination clearly decreases with time.
If booster vaccinations have lapsed, a single injection is considered adequate if the interval
since the last vaccination is less than three years, but if it is more than three years, two
vaccinations should be considered to ensure that optimal protection is provided.
Boosters using FHV vaccines produced by another manufacturer are acceptable.
Cats that have recovered from disease associated with FHV may not have lifelong protection
against further episodes of disease. Furthermore, in most cases, definitive identification of the
infectious agent involved in diagnostic testing will not usually have been undertaken and the
cat may be susceptible to infection with other respiratory tract pathogens. Therefore,
vaccination of recovered cats is generally recommended.