Final answer:
The child can receive both the MMR and varicella vaccines today, as more than 3 months have passed since the completion of chemotherapy and only a low-dose topical steroid has been used, which typically does not require delaying live vaccines.
Step-by-step explanation:
Given the child's history of Acute Lymphoblastic Leukaemia (ALL) and recent chemotherapy, along with the use of a low-dose topical steroid for eczema, the timing for administering live vaccines, such as MMR and varicella, should be carefully considered. According to the Centers for Disease Control and Prevention (CDC) guidelines, live vaccines should be withheld for at least 3 months after completion of chemotherapy. Since the child's last chemotherapy session was 6 months ago, this waiting period has been satisfied. However, the use of systemic steroids at immunosuppressive doses is another consideration for delaying live vaccines. In this scenario, it is only a low-dose topical steroid being used, which typically does not contraindicate the administration of live vaccines.
As long as the child is not currently receiving systemic steroids or other immunosuppressive treatments, and since they have been off chemotherapy for more than 3 months, he may receive both the MMR and varicella vaccines today. However, if there is any uncertainty regarding the strength and duration of topical steroid use, or if the child has any other immunocompromising conditions, it is best practice to consult with a healthcare provider for the most appropriate advice.
Therefore, the correct answer to when the child may receive his MMR and varicella vaccines, based on the provided information and in the absence of other contraindications, is: both today.