Definition of AD and AD phenotype
Children were stratified into early childhood (0–3 years), preschool (4–6 years), and school-age (7–9 years) groups. AD was considered present if a diagnosis was constituted at any period within the specified age group. Current AD was determined as a diagnosis within the age group and the presence of AD symptoms confirmed by a physician within the previous 12 months. Our study targeted various phenotypes, including the: early-transient, defined as AD onset within 2 years of age and no further symptoms; early-persistent, defined as AD onset within 2 years of age and symptoms not improving within 2 years; and late-onset, defined as AD onset after 2 years of age(13).