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Correspondence to Dr Eva Louise Wooding; [email protected]
Introduction
Chickenpox, or varicella, is the primary infection of varicella zoster virus (VZV), a double-stranded DNA virus in the Herpesviridae family. Varicella primarily affects children. Following primary infection, VZV lays dormant in sensory nerve ganglia, and later may reactivate leading to its second clinical syndrome, herpes zoster, also known as shingles. The WHO estimates there are 84 million cases of varicella annually, with 950 000 associated disability-adjusted life years attributed to VZV in 2019 and 14 553 deaths.1 Complications are primarily localised in the skin, lungs and brain, including secondary bacterial infections of varicella lesions, varicella pneumonitis or secondary bacterial pneumonia, cerebellar ataxia, encephalitis and stroke.2 Morbidity and mortality are more common in immunocompromised patients.3 This review will discuss the burden of varicella in children, outline current vaccine programmes and their impact, and consider the rationale for universal implementation.
Background
Epidemiology
In Europe, the annual burden of varicella in an unvaccinated population is estimated at 5.5 million cases. An estimated 0.82 severe cases per 100 000 children annually were described in a UK surveillance study during 2002–2003, including secondary infection, organ impairment and death.4 Humans are the only reservoirs of VZV, and the virus has a single known serotype. In temperate regions, varicella acquisition is most common during winter to spring with peak incidence between 3 years and 6 years of age. In tropical regions, transmission is highest in the cool, dry months, with large outbreaks occurring every 2–5 years.1 Exposure to varicella occurs earlier in urban, temperate environments, and in children attending formal childcare settings, where exposure is greater. Varicella is highly contagious with an estimated R0 of 3.3–16.9 across 11 European countries.5
If infection is introduced, an estimated 61%–100% of susceptible household contacts would develop varicella. Seroprevalence data from other tropical and island populations corroborates that later acquisition of varicella immunity occurs in these groups. In Saint Lucia, a seroprevalence study of 1810 people found that 90% of the population studied remained susceptible to varicella at 15 years of age, a figure that would be unthinkable in most temperate countries.6 90% of people in the USA were found to be seropositive for varicella by 20 years...