Individualised Dose Optimisation of Ganciclovir in Immunocompromised Children Trial (ID-MAGIC)

Inclusion Criteria: 1. Immunocompromised patients including transplant recipients (haematopoietic stem cell transplant (HSCT), solid organ transplant (SOT)), those receiving chemotherapy or other immunosuppression or those with a known/suspected inborn error of immunity (determined by an immunologist); and 2. Detectable clinically significant CMV viraemia and treating clinician determines that antiviral therapy is indicated. 3. Willing to partake in the trial 4. Willing/able to attend all follow up visits and capable of completing all trial assessments. 5. Legally acceptable parent/guardian capable of providing consent on the participant's behalf. 6. Treating clinician agreeable to child being enrolled in the trial. Exclusion Criteria: 1. Current or prior CMV infection with documented genotypic resistance to GCV (UL97 and/or UL54); or 2. Severe renal impairment (defined as estimated glomerular filtration rate (eGFR) \<25mL/min); or 3. Congenital CMV infection; or 4. Life expectancy of less than 7 days as determined by the treating physician; or 5. History of allergy, or adverse reaction to GCV, aciclovir or any component of the formulation; or 6. Treating clinician determines that combination antiviral therapy is indicated for CMV infection; or 7. Has received \>3 days of IV GCV or foscarnet or oral valganciclovir for the treatment of CMV infection prior to enrolment; or 8. Prior enrolment in the trial; or 9. Current recipient of another investigational product used for the treatment of CMV infection, as part of a clinical trial.