Varicella Health Article

Epidemiology

The epidemiology of varicella differs in temperate and tropical climates. ^5,63,64 In most temperate climates more than 90% of people are infected before adolescence, ^8,9,65,66 whereas in many tropical climates the disease is acquired later in life and adults are more susceptible than are children. ^63,64,67,68 Epidemiological variation might relate to differences in population density and risk of exposure, differences in transmissibility of the heat-labile varicella-zoster virus in hot, humid conditions, environmental and social factors, or a combination of all these factors. Varicella shows pronounced seasonality in temperate climates and most tropical climates, with peak incidence in the cooler, drier months during winter or spring. ^63,69–72 In temperate climates studies have shown that disease incidence in the total population is in the range of 13–16 cases per 1000 people per year, ^3,83,96 with substantial year-to-year variation. ^5,7,69 Epidemics tend to arise at intervals of 2–5 years. ^69,70,72

Varicella is a childhood disease, with the highest incidence in children aged 1–9 years. ^{4,5,64,69,73–79} Over the last decade, a shift to younger age at infection (below 5 years) has been observed, probably because of attendance at child-care centres. ^5,77,80 Investigators have also examined sex (most report no differences in seroprevalence between male and female individuals); number of siblings (reduced susceptibility in children with more siblings); and race (black adolescents were reported to be more susceptible than white adolescents in the USA). ^8,65,81

Older age and a compromised immune system are the most important risk factors associated with severity of varicella disease and death. Anecdotal evidence from case reports and case series suggests that varicella in pregnant women is more severe than in non-pregnant women. Population-based prospective studies are needed to investigate this possibility. ⁸² In developed countries, average crude varicella mortality rates range from 0.3 to 0.5 per million population, and overall case fatality rates are about 2–4 per 100,000 cases. ^{7,13,69,74,83–85} The risk of dying from varicella is highest at the extremes of age: in adults, the risk of death was 23–29 times higher, and in infants four times higher, than that in children, in whom case fatality rates were about one per 100,000. ^7,13,74,83

Crude rates of admission to hospital with varicella in developed countries range from about two to six per 100,000 population. ^{69,74,83,84,86,87} Most of these admissions (56–67%) were children, which is consistent with the fact that 90% of varicella cases happen in this age group. ^3,5,69 For all ages combined, rates of hospital admission per 1000 cases of varicella have ranged from 2.2 to 4.7 in national studies, mainly in France, USA, and UK ^7,83,86,87 Variability in surveillance, health-care systems, and completeness of ascertainment could account for these reported differences. As with the risk of dying from varicella, risk of hospital admission is higher for infants and adults than for children. ^{5,73,74,83,86}

Although varicella infection is more severe in immunocompromised people (mortality was about 7% in children with acute lymphocytic leukaemia before the availability of varicella zoster immunoglobulin and effective antiviral agents), ⁵⁶ most cases of severe morbidity and mortality are seen in healthy people. ¹³ For example, 70% of all varicella deaths in France (1990–97) took place in people with no underlying high-risk medical disorders (including HIV/AIDS, leukaemia and other malignant diseases, other forms of blood dyscrasia, and immune deficiencies). ⁸³ Similarly, in the USA (1970– 94), 89% and 75% of varicella deaths in children and adults, respectively, occurred in otherwise healthy people. ¹³ Almost 90% of people admitted to hospital with varicella are described as healthy or immunocompetent. ^14,86

Population-based data for mortality and hospital admissions from developing countries and from those with tropical climates are sparse. The high age of infection and severe disease in adults in tropical countries might account for the increased morbidity and mortality from varicella and its complications, such as congenital varicella syndrome, in such countries. ^62,88,89 We need more population-based studies to investigate varicella morbidity and mortality in countries with high HIV transmission and prevalence of AIDS. ^62,88