Nearly all children in the UK catch chickenpox by the age of 10. In most cases the disease is mild, but some children develop complications which can be serious. (See 'What are the symptoms?' above.)
Chickenpox tends to be more severe in adults than in children. More adults are hospitalised as a result of chickenpox complications, often with a serious form of pneumonia. However, most adults who are otherwise healthy make a full recovery with good medical care.
The risk of chickenpox infection leading to serious complications is highest in the following groups:
Pregnant women who are not immune to chickenpox are at risk from a type of life-threatening pneumonia if they catch chickenpox, especially in the later stages of pregnancy. About 3 in every 1000 pregnant women in the UK catch chickenpox. Between 1985 and 1998, nine pregnant women died in the UK from chickenpox complications. Their unborn babies are also at risk from a rare condition called foetal varicella syndrome (FVS). This can result in serious long-term damage to the baby or even death, particularly if the mother catches chickenpox in the first 20 weeks of pregnancy. (Pregnant women who have had chickenpox are not at risk from chickenpox infection, and neither are their unborn babies.)
Babies around the time of birth (a week before birth to a week after birth) are at risk from a rare condition called disseminated varicella, in which the chickenpox virus spreads to internal organs. This is extremely serious and the death rate is high. They are also at risk of other complications including pneumonia. The risks are highest for the infant if their mother develops chickenpox in the week before birth or in the first few days after birth, rather than if the baby catches it from another infected person after birth. This is because the baby is exposed to a bigger amount of the virus if it is their mother who passes the infection to them across the placenta.
Smokers who are not immune to chickenpox are at higher risk from a life-threatening form of pneumonia if they catch chickenpox.
People who do not have a fully-working immune system (for example, those with HIV, those without a spleen and those receiving chemotherapy treatment) are at risk from disseminated varicella, in which the chickenpox virus spreads to internal organs. This is extremely serious and can cause death. They are also at risk of other complications including pneumonia, meningitis and septicaemia (severe blood poisoning).