Varicella vaccination—a critical review of the evidence

Varicella (chickenpox) is an universal, highly infectious disease characterised by a pruritic vesicular eruption associated with fever and malaise caused by varicella zoster virus (VZV).In children, the illness is usually self limiting, lasting four to five days, but at least 1% of children under 15 years experience a complication.1 2 These include secondary bacterial infection (particularly with group A beta haemolytic streptococcus), 3 pneumonia, encephalitis, haemorrhagic complications, hepatitis, arthritis, and Reye syndrome.4 Furthermore, 10-50% of all children will visit a physician with an infection.[5][6][7] The mortality rate of varicella in children under 14 years in the United States is estimated at 2 per 100 000 cases, 8 and 90% of these have no risk factors for severe disease.9 Adults experience only 5% of all varicella cases, but experience more severe disease (hospitalisations 18 per 1000) and deaths (50 per 100 000).10 Herpes zoster (shingles), a painful, dermatomal, vesicular rash occurs with reactivation of the virus in approximately 15% of the population.11 The likelihood of developing herpes zoster increases with advancing age: the incidence is approximately 74 per 100 000 children aged under 10 years, 11 300 per 100 000 adults aged 35-44 years, 12 and 1200 per 100 000 adults over 75 years.12 In temperate climates, 95% of varicella cases occur among persons less than 20 years of age.13 14 Seropositivity is lower in adults from tropical and subtropical areas.15 16 Seronegativity in adults may be increasing in temperate populations, as shown by a significant upward trend in age distribution of chickenpox cases in England and Wales, 17 and increasing varicella susceptibility in young US adults.18 A live attenuated varicella vaccine was first developed in 1974 in Japan by Takahashi and colleagues.19 As this Oka strain virus is heat sensitive, Biken/Oka vaccine (Japan) and Varivax (Oka/Merck) require storage at -15°C and administration within 30 minutes of reconstitution to retain potency (product monograph).Oka strain vaccines were first licensed for use in high risk children in Europe in 1984 and Japan in 1986.Licensure for use in healthy children commenced in 1986 in Japan, 1988 in Korea, and most recently in the USA, Sweden, and Germany (1995), 20 21 and Canada (December 1998).22 Many millions of doses have been given in total. Aims of reviewThe purpose of this review was to evaluate the evidence that bears on the various options for use of vaccine to prevent varicella in healthy individuals.These include universal vaccination of healthy infants, catch up vaccination of older children, and vaccination of susceptible adolescents and adults.Models of cost eVectiveness and epidemiological change suggest that implementation of routine varicella vaccination for infants and children could reduce total number of cases and case severity, and generate cost savings.23 Potential harm that may occur as a result of vaccination includes immediate adverse reactions, transmission of varicella from vaccinees, an increased risk of zoster, and a shift in varicella cases to an older age group (and hence more severe disease).24 In evaluating varicella vaccine it is important that these issues are considered in addition to vaccine eVectiveness. Methodology of searchMEDLINE was searched from 1966 to December 2000 using the MeSH subheadings chickenpox, vaccination, and human (search date 19 January 2001).There was no language restriction.Methodological search terms included: random allocation, placebo, doubleblind method, comparative study, epidemiologic methods, research design, clinical trials, controlled clinical trials, meta-analysis, drug evaluation, prospective studies, and evaluation studies.EMBASE was searched using a similar strategy.To identify other studies, we searched reference lists of located studies; the Internet for position papers and summaries from health organisations such as the World Health Organisation and the Centers for Disease Control and Prevention; vaccine product information; and the Cochrane Library.Published studies were included if they: (1) considered healthy, human subjects vaccinated with VZV vaccine; and (2) were controlled trials addressing the incidence of varicella, zoster, or adverse outcomes.Prospective cohort studies were considered only for longer term outcomes of varicella and zoster following vaccination.To limit the analysis to studies with the highest methodological quality, prospective cohort studies were excluded if: (a) they contained less than 50 subjects; (b) loss to follow up was not described; or (c) duration of follow up was less than one year.All eligible

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