The epidemiology of this infection appears to be similar in all developed countries with temperate climates. Epidemics occur each winter/spring, with some variability from year to year. Approximately 3 percent of each birth cohort are infected during infancy. Thereafter, 8–9 percent of the birth cohort are infected each year throughout childhood, so that by age 10 years fewer than 15 percent, and by age 14 years fewer than 10 percent, remain susceptible. The infection rate drops rapidly in adolescence and young adulthood to about 1 percent per year. By age 40 years almost the entire birth cohort (over 97 percent) have been infected, so that only a few adults remain susceptible.
Transmission of the virus is less efﬁcient in tropical climates. Adolescent and adult immigrants to New Zealand from such countries are more likely to be susceptible, placing them at risk of contracting chickenpox in their new environment. Being older, they are more likely to suffer severe disease.
The characteristics of infection appear to conspire at times to maximise the disruption to families. If a child is exposed outside the home, by the time the rash occurs the child will have been infectious for two days. This means that any susceptible household contacts will become unwell just as the first child is starting to recover. This results not only in morbidity but also in financial consequences for parents missing work.
Varicella vaccine has been introduced into childhood immunisation programmes overseas, including the US from 1995 and Australia from 2005, with dramatic reductions in varicella morbidity, hospitalisations and mortality.1–3 Following the introduction of varicella vaccine onto the childhood schedule, exposure to wild-type virus decreases, and therefore adults are less likely to boost immunity to latent HZ. It is hypothesised that lack of boosting may lead to an increase in HZ in older adults.
However, a study in the US4 from 1992 to 2002 has shown that although the incidence of varicella decreased in children (from 2.63 cases per 1000 person-years in 1992 to 0.92 cases per 1000 person-years in 2002), there was no increase in HZ in adults of any age: the age-adjusted rate of HZ was 4.05 cases per 1000 person-years in 1992 and 3.7 cases per 1000 person-years in 2002. Studies from the UK and Canada have reported increases in HZ not associated with a vaccination programme, and some US data showed HZ rates were increasing prior to the initiation of their varicella vaccination programme.5, 6 It is inevitable that HZ rates will rise in New Zealand as the population ages.
It remains unclear whether the introduction of childhood mass VZV vaccination does significantly alter the epidemiology of HZ. Studies that have investigated this issue have been unable to attribute any increase in incidence of HZ to the childhood VZV vaccine programme.7, 8
In New Zealand it is expected that 90 percent of children will have had varicella infection before adolescence, with peak incidence in the 5–9 years age group. With higher participation in early childhood services, a greater proportion of infections may now be occurring in preschool-aged children.
As varicella is not a notifiable disease, accurate data collection is limited for uncomplicated varicella, and hospital discharge data depends on accurate coding. This may result in under-reporting of complications secondary to varicella infection.
Hospital discharge information for varicella between 1970 and 2013 is shown in Figure 21.1. The rate of hospital discharges for the 0–4 and 5–9 years age groups was higher compared with older age groups because the disease is most common in childhood. However, adults, adolescents and infants are more likely to suffer severe illness or the complications of chickenpox.9
Source: Ministry of Health
Based on overseas rates, it is estimated that up to one case of congenital varicella syndrome may be expected in New Zealand each year, although few have been reported.
A retrospective survey of admissions to the paediatric intensive care unit (PICU) at Auckland’s Starship Hospital (2001–2011) found 26 children admitted for varicella or its secondary complications.10 The main PICU admission reasons were neurological (38.5 percent) and secondary bacterial sepsis or shock (26.9 percent). Four children died (15 percent), three of whom were immune compromised. A further eight children (31 percent) had ongoing disability at discharge, most having had no prior medical condition.
In summary, in a typical year New Zealand is estimated to experience approximately 50,000 chickenpox infections, of which several hundred result in hospitalisation, one to two cases result in residual long-term disability or death, and 0.5–1 cases result in severe congenital varicella syndrome. About two-thirds of this burden is borne by otherwise healthy children, and less than one-tenth by children with a disease associated with immunosuppression. Approximately one person per year dies from VZV, and most of the VZV-associated deaths occur in adults.