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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

1. Preblud SR, Orenstein WA, Bart KJ. Varicella: clinical manifestations, epidemiology and health impact in children. Pediatric Infectious Disease 1984; 3:505-9.
2. Guess HA, Broughton DD, Melton LJ, 3rd, Kurland LT. Population-based studies of varicella complications. Pediatrics 1986; 78:723-7.
3. Brody MB, Moyer D. Varicella-zoster virus infection. The complex prevention-treatment picture. Postgraduate Medicine 1997; 102:187-90, 192-4.
4. MacIntyre CR, Chu C, Burgess MA. Use of hospitalisation and pharmaceutical prescribing data to compare the prevaccination burden of varicella and herpes zoster in Australia. Epidemiology & Infection. 2003; 131:675-82.
5. Chant KG, Sullivan EA, Burgess MA, et al. Varicella-zoster virus infection in Australia. Australian & New Zealand Journal of Public Health 1998; 22:413-8.
6. Brotherton J, McIntyre P, Puech M, et al. Vaccine preventable diseases and vaccination coverage in Australia 2001 to 2002. Communicable Diseases Intelligence 2004; 28 (Suppl 2):S1-S116.
7. Bowsher D. The lifetime occurrence of Herpes zoster and prevalence of post-herpetic neuralgia: A retrospective survey in an elderly population. European Journal of Pain:Ejp 1999; 3:335-342.
8. Lin F, Hadler JL. Epidemiology of primary varicella and herpes zoster hospitalizations: the pre-varicella vaccine era. Journal of Infectious Diseases 2000; 181:1897-905.
9. Vessey SJ, Chan CY, Kuter BJ, et al. Childhood vaccination against varicella: persistence of antibody, duration of protection, and vaccine efficacy. Journal of Pediatrics. 2001; 139:297-304.
10. Tugwell BD, Lee LE, Gillette H, Lorber EM, 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

 

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