This study is being conducted at seven major children's hospitals in Australia and New Zealand to test a new approach for treating a virus, called cytomegalovirus in children with weakened immune systems. The researchers want to find out if using a web app to customise the dose of a medication called ganciclovir is better at clearing the virus over a six-week period compared to the standard method of giving the medication.

Immunocompromised children between 1 months to 18 years with cytomegalovirus viraemia who are admitted to one of the participating sites will be enrolled into the trial if eligible (see eligibility criteria) and randomly allocated into two groups. Children in the 'control- standard dosing group' will receive standard intravenous ganciclovir treatment for cytomegalovirus viraemia at a standard dosing of at 5mg/kg IV BD. Children in the "intervention: individualised dosing using a web app group" will receive a personalised intravenous ganciclovir dose calculated using an individualised IV ganciclovir dosing app. This approach considers the patient's weight, creatinine level, and target drug exposure, allowing for tailored dosing based on individual pharmacokinetic parameters. The virological clearance by 6 weeks of the children in each of the two groups will be compared.

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.

Beginning 12 months following analysis and article publication, upon approval of the request, the following will be made available long-term for use by future researchers from a recognised research institution whose proposed use of the data has been ethically reviewed and approved by an independent committee and who accept MCRI's conditions for access:

• Individual participant data that underlie the results reported in this article after deidentification (text, tables, figures, and appendices)
• Trial protocol, Statistical Analysis Plan, PICF

Beginning 12 months following analysis and article publication, upon approval of the request

Researchers must be from a recognised research institution whose proposed use of the data has been ethically reviewed and approved by an independent committee and who accept MCRI's conditions for access