Varicella-zoster (chicken pox) vaccines for Australian children:
information for GPs and immunisation providers
Varicella-zoster virus (VZV)
VZV is one of eight
herpes viruses that cause infections in humans. It is a double-stranded
DNA virus and is most closely related to herpes simplex virus types 1 and
2. These viruses rapidly proliferate, invade and destroy infected cells.
Like other herpes viruses, VZV has the unusual ability to establish a
latent infection in nerve ganglions, which can later reactivate causing
shingles (herpes zoster).
Epidemiology and burden of varicella and herpes zoster
In unvaccinated populations, varicella
(chickenpox) is primarily a childhood illness with more than 90% of the
population in temperate countries developing clinical or serological
infection by adolescence.1 Varicella is
generally a benign, self-limiting illness in children. Severe illness
causing hospitalisation, or even death, becomes more likely with
increasing age,2 or with a
suppressed immune system.3 There are
about 240,000 cases, 1,500 hospitalisations and 7 deaths each year from
varicella in Australia.4,5,6 Although the
risk of severe disease is greater in adolescents and adults, the greatest
absolute number of hospitalisations are in children because disease
incidence is far higher in childhood.
Herpes zoster (HZ) or ‘shingles’ is a sporadic disease, caused by the
reactivation of latent VZV. It is usually self-limiting and is
characterised by severe dermatomal pain. This pain can persist
(post-herpetic neuralgia), especially in the elderly.7 Although HZ
can occur at any age, incidence increases with age (in contrast to
chickenpox) and most cases occur after the age of 50.8
Vaccine efficacy and recommendations for use
vaccines containing live attenuated (weakened) varicella-zoster virus are
currently available in Australia (Varilix and Varivax Refrigerated).
Detailed information about them is available on page 280 of the Australian
Immunisation Handbook (AIH). These vaccines are derived from distinct
genetic variants of the Oka varicella-zoster virus strain.
Varicella vaccination is recommended for Australian children at the age
of 18 months and will be funded from November 2005. Vaccine efficacy in
children is reported to be 88-98% from clinical trials,9 but vaccine
effectiveness measured in outbreaks has ranged from 44% to 100%.10 Younger age
at vaccination (below 15 months) appears to increase the risk of vaccine
failure, probably because maternal antibodies are still present in some
children at 12-15 months, reducing immunogenicity of the vaccine.11 This is one
reason why the vaccine is recommended at 18 months of age in
The response to a single dose of varicella vaccine decreases as age
increases. Healthy adolescents (14 years and older) and adults require two
doses, 1-2 months apart.12 Vaccination
is recommended for all adolescents and adults who are not already immune.
It is particularly recommended for those at high risk of exposure to, or
complications from, varicella, such as health care workers, child care
workers, non-immune women before pregnancy and parents. Vaccination of
non-immune household contacts of immunosuppressed persons is also
important to prevent transmission of varicella to the immunocompromised
person (see page 284 AIH). As part of the funding for Varicella
vaccination, children aged 10-13 years who have not received the vaccine
or who have not had the disease are eligible for free vaccine from
November 2005, as part of a long-term catch-up program.
Varicella vaccines are safe
to administer at the same time as all other recommended vaccines on the
schedule (given subcutaneously at a separate site). Other live vaccines
(eg. MMR) should either be administered at the same time or at least four
weeks apart. There is evidence of higher vaccine failure rates when MMR is
not given simultaneously with varicella but within 4 weeks.13 Serology is
not necessary prior to vaccination, as vaccination of individuals who are
already immune to varicella is well tolerated.
Vaccination is contraindicated in
pregnancy, and pregnancy should be avoided for one month following
vaccination. However, in women inadvertently vaccinated during pregnancy,
no adverse effects have been reported. The vaccine is also contraindicated
for immunodeficient persons, but their household contacts should be
vaccinated if non-immune, to protect the immunodeficient person against
infection. Previous anaphylactic reaction to neomycin is a
contraindication to both vaccines and gelatin anaphylaxis is a
contraindication to Varivax Refrigerated.
Vaccine reactions are generally
mild, and include fever and injection site reactions (7-30%). Rashes
(localised or generalised) following vaccination may be due to either
coincident wild VZV infection or be related to the vaccine virus. The
latter tend to occur later following vaccination (median of 21 days for
vaccine virus vs 8 days for VZV; see page 285 AIH). If a rash develops,
vaccinees should avoid contact with immunosuppressed persons, although
virus transmission is extremely rare and most rashes after varicella
vaccination are due to other causes, especially in children. More serious
adverse events occurring soon after vaccination have been reported at a
rate of 2.9 per 100,000 doses by passive surveillance. A causal, as
opposed to coincidental, relationship to vaccine is not established, but
is plausible, for anaphylaxis following vaccination and for
thrombocytopenia, ataxia and encephalitis, as these are rare complications
of natural varicella infection.14
Advice to parents
Varicella vaccine is recommended
to prevent both chickenpox and the lifetime risk of shingles due to
chickenpox infection. About 75% of Australian children will have chicken
pox by the age of 10 and there are about 1000 hospitalisations a year in
children aged 10 and under. Chickenpox poses particular dangers for
certain members of the community, such as pregnant women and
immunosuppressed people. The main benefit of vaccination for individual
families is avoiding time off work caring for infectious children required
to stay at home. The vaccine is a live virus, which is well tolerated in
most people. It is likely to be up to 90% effective at preventing
chickenpox when given at the recommended age of 18 months. Cases that
occur despite vaccination are usually mild. Choosing not to vaccinate with
varicella vaccine does not currently impact on immunisation status for
childcare and other payments or school registration.
NHMRC 2003. The Australian Immunisation Handbook 8th edition.
Varicella-Zoster p 278-90 www.immunise.health.gov.au/handbook.htm
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11. Galil K, Fair E,
Mountcastle N, Britz P, Seward J. Younger age at vaccination may increase
risk of varicella vaccine failure. Journal of Infectious Diseases. 2002;
12. Kuter BJ, Ngai A, Patterson CM, et
al. Safety, tolerability, and immunogenicity of two regimens of Oka/Merck
varicella vaccine (Varivax) in healthy adolescents and adults. Oka/Merck
Varicella Vaccine Study Group. Vaccine 1995; 13:967-72.
13. CDC. Simultaneous administration of varicella vaccine and
other recommended childhood vaccines – United States, 199501999. MMWR Morb
Mortal Wkly Rep 2001;50:1058-1061.
14. Wise RP,
Salive ME, Braun MM et al. Postlicensure safety surveillance for varicella
vaccine. JAMA 2000;284:1271-9.
© Helen Quinn, 18 May 2005