For the majority of children who are immunocompromised or who require immunosuppression, their primary diagnosis necessitates a difficult programme of prolonged treatment. In countries with no varicella vaccination, this is compounded by the ever-present and significant threat of chickenpox. Potential exposures must be avoided, causing children to miss school and playgroups whenever chickenpox is at large. Each contact with varicella results in negotiating blood testing to assess their current varicella zoster virus (VZV) status followed by either varicella zoster immunoglobulin (VZIG) therapy or antiviral treatment in hospital or five times a day at home. Immunosuppressive treatment may be interrupted with consequent risk of reduced remission rates, or disease relapse.

Before the availability of VZIG and antiviral agents, the death rate from varicella was around 7% for children being treated for cancer, with 32% having visceral dissemination; children with lymphopenia were at highest risk.1 Since the use of prophylaxis and treatment, deaths from varicella are now very rare.2 However, without careful management of varicella exposure, there remains a risk of severe disseminated disease, with the development of varicella pneumonia, encephalitis, hepatitis, or haemorrhagic complications.3 Despite the best preventive interventions, child deaths still occur.4

The number of immunocompromised children is unknown but is thought to be increasing, including children with primary immunodeficiencies, acquired immunodeficiency and those that are immunosuppressed by treatment including novel biological therapies. This is not a homogenous population; those with impaired T cell function are particularly susceptible to severe varicella.

Although antibody responses to varicella are critical in the control of acute infection, T lymphocyte mediated immunity (comprising both CD4 and CD8 effector and memory T cells) provides the antigen-specific adaptive immunity which stops viral replication and is essential for recovery from …