Varicella Health Article

Varicella Vaccination Programmes

Varicella vaccine has now been licensed and is widely available throughout the world. The goal of a vaccination programme determines the strategy used. Because varicella is mainly a childhood disease, prevention of varicella and its attendant morbidity and mortality is best accomplished through universal vaccination of children. Targeted programmes (eg, for health-care workers, household contacts of immunocompromised people, susceptible adolescents or adults) offer either direct or indirect protection to high-risk individuals in the population. A vaccination programme for susceptible adolescents and adults could potentially prevent about 30% of varicella deaths and hospital admissions if high coverage could be achieved in these age-groups. ^173,174

The USA has a universal varicella-vaccination programme and other countries have added varicella vaccine to their childhood immunisation schedule—eg, Uruguay, Qatar, parts of Italy and Israel, Taiwan, Germany, Australia, Canada, and South Korea. Many European countries, such as Switzerland, recommend the vaccine for specific risk groups such as immunocompromised people, health-care workers, and susceptible adolescents and adults. Availability of a combination MMRV vaccine, recently licensed in the USA, Germany, and Australia and expected in other countries soon, might simplify implementation of childhood varicella vaccination programmes in countries considering such programmes. In developing countries, the health burden caused by other diseases is higher than that of varicella, so varicella vaccination is a low priority for introduction into their national immunisation programmes. ¹⁷⁵ However, more countries should consider recommending varicella vaccine for health-care workers. ¹⁷⁶

A programme of varicella vaccination has the potential to change the epidemiology of herpes zoster as well as varicella. Immunocompromised children who are vaccinated against varicella have a reduced risk of developing herpes zoster, ¹⁶⁵ as do healthy vaccinated children. ¹⁷⁷ However, data from longer follow-up studies are needed to see if these lower risks are sustained throughout life. ¹⁷⁷ Mathematical models have predicted that, in the long term, as vaccinated children aged into adulthood, occurrence of herpes zoster in the population would fall. ¹⁷⁸ However, in the short and medium term (20–60 years), such models have predicted an increase of herpes zoster in the population. ^178,179 In Canada, the USA, and the UK, rates of herpes zoster were increasing before varicella vaccination programmes, which complicates the interpretation of possible changes in herpes zoster epidemiology. ^180,181 Although there are reports that herpes zoster has increased in the USA in recent years, ^182,183 neither study compared their data with pre-vaccine baseline incidence rates for herpes zoster. One of the studies attributed the increased incidence in 10–17 year olds to increased use of steroids. A different study suggests that the increase in herpes zoster in the USA began before the universal vaccination programme was implemented in 1995, ¹⁸⁴ and yet another study, that had baseline data available, did not find an increase in herpes zoster between 1992 and 2002. ¹⁸⁵

Postvaccine Epidemiology

In the USA, since the introduction of the childhood varicella vaccination, varicella cases, hospital admissions, and deaths have fallen by more than 80% in children, and to a lesser extent in adults and infants, who are protected by indirect effects (so-called herd immunity). ^{181,186–189} By 2000, when vaccine coverage in young children ranged from 74–84%, varicella cases were reduced by 71–84% in active surveillance sites. ¹⁸⁶ By 1999–2001, the rate of deaths in which varicella was the underlying cause was 92% lower in children aged 1–4 years than in the 5 years preceding the vaccination programme (1990–94). ¹⁸⁸ Over the same period the varicella mortality rate was reduced by 74–89% in infants younger than 1 year and people aged 5–49 years. ^187,188 The equivalent drop in varicella deaths in adults older than 50 years was 16%, which is consistent with the low positive predictive value of varicella on death certificates in this age group in the USA. ¹⁸⁹ By 2002, hospital admissions due to varicella had fallen by 88%, compared with rates in 1994–95 (figure 4 ). ¹⁹⁰ By 2004, vaccine coverage in children 19–35 months of age had reached 88% nationally. ¹⁷⁷ Despite these successes, varicella outbreaks have been reported lately in highly vaccinated school populations. ^146,153 These outbreaks have been smaller than those in the pre-vaccine era. However, along with concern that improved protection is needed for the 15–20% of vaccinees who are not fully protected after one dose, they have prompted consideration of a two-dose policy for varicella vaccine. ^153,191