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 (shingles).
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
Two 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 Australia.
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.
Vaccine administration
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.
Contraindications
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.
Adverse events
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.
References
NHMRC 2003. The Australian Immunisation Handbook 8th edition. Varicella-Zoster p 278-90 www.immunise.health.gov.au/handbook.htm
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in an elderly population. European Journal of Pain:Ejp 1999; 3:335-342.
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and vaccine efficacy. Journal of Pediatrics. 2001; 139:297-304.
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Hedberg K, Cieslak PR. Chickenpox outbreak in a highly vaccinated school
population. Pediatrics 2004;113:455-59.
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; 186:102-5.
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