Inclusion & Exclusion Criteria

1. Aged 30 days (post-natal age) to 15 years inclusive, and below the country-specific age of consent for the duration of the trial.

2. KD defined in at least one of the three following ways:

a. As per American Heart Association (AHA) criteria [1]: namely fever for at least 5 days in addition to 4 of the following 5 clinical criteria:

i. bilateral non purulent conjunctivitis
ii. cervical lymphadenopathy
iii. polymorphous skin rash
iv. changes in lips or mucosa (strawberry tongue, red cracked lips, diffuse erythematous oropharynx)
v. extremity changes (erythema, oedema of palms and soles in initial phase, and at convalescent stage skin peeling)

b. OR less than 5 days of fever but all 5 clinical criteria above

c. OR incomplete KD cases, as per a modified*AHA definition [1], namely:

i. children/adolescents (>1 year old) with fever greater than or equal to 5 days AND at least 2 other compatible clinical criteria as listed above; OR infants ≤ 1 year old with fever greater than or equal to 7 days without other explanation;

AND for both age groups:
ii. CRP ≥30 mg/L or erythrocyte sedimentation rate (ESR) ≥40 mm/hr (or both)

AND for both age groups:
iii. EITHER the presence of any 3 or more of: anaemia for age (haemoglobin < lower limit of normal reference range for local laboratory) platelet count ≥450 x10⁹/L or upper limit of normal reference range for local laboratory); white cell count ≥15 x10⁹/L; urine ≥10 white blood cells per high power field
iv. OR abnormal echocardiogram compatible with KD but without established CAA, with ≥ 3 of the following suggestive features: decreased left ventricular function, mitral regurgitation, pericardial effusion, or dilated but non-aneurysmal coronary arteries (internal diameter 2≤Z

3. Written informed consent from appropriate legal representative(s), and assent from patients who have not reached the age of consent and will not reach the age of consent for the duration of the trial in the participating country, but are judged to have capacity for this (depending on both age and acuity of illness).

*This definition of incomplete KD is modified from the AHA definition by firstly, the exclusion of aneurysmal coronary artery changes as the sole echo finding, since this is an exclusion criterion for KD-CAAP, and secondly the inclusion of low platelet count as well as high platelet count, as highlighted in recent European consensus SHARE guideline [5].

Note that patients with KD can still be included in KD-CAAP if they have started IVIG treatment, as long as they are randomised no more than 24 hours after the IVIG infusion is initiated (see exclusion criteria below).

Test results must be from tests done on the calendar day of randomisation or the day before.

Disease-related exclusions*:

1. This diagnosis is a second or further episode of KD.
2. Already established CAA at screening.
3. Severe Congestive Heart Failure or cardiogenic shock defined as the presence of hypotension and shock requiring the initiation of volume expanders.
4. Known congenital coronary artery abnormality that would impair assessment of the primary endpoint.
5. Suspected macrophage activation syndrome.

Exclusions related to medications:

6. Started IVIG more than 24 hours prior to randomisation.
7. Known hypersensitivity to prednisolone or methylprednisolone, or known phenylketonuria to aspartame used in a formulation in an infant less than 12 weeks.
8. Current oral, intravenous or intramuscular corticosteroid treatment for more than 3 days in previous 7 days prior to randomisation.
9. History of previous severe reaction to any human immune globulin preparation.

Exclusions related to general health or other issues:

10. Active varicella zoster virus or influenza infection; or known exposure to a case of varicella within the previous 21 days prior to randomisation if known to be non-immune.
11. Co-enrolment in another study/trial of an investigative medicinal product.
12. Pregnant or/and breastfeeding adolescents^†.

*Disease-related exclusions relate to those (rare) patients who already have severe fulminant inflammation and/or shock when they are diagnosed with KD, in whom recent European consensus suggests corticosteroids and/or other immunosuppression are required [5]. Such exceptional cases represent a small minority and therefore will not substantial impact on recruitment targets.

†A blood or urine pregnancy test must be completed on the day or day before randomisation for adolescents who have begun menstruation.