Atopic dermatitis (AD), also known as atopic eczema, is a common chronic inflammatory skin disorder, with an increasing global incidence in the past few decades ^[1][2][1,2]. The pathophysiology of AD is complex and multifactorial, involving genetic disorders, skin barrier dysfunction, aberrant immune responses, and skin microbial dysbiosis. AD often develops during early infancy and childhood, and is characterized by eczematous lesions on the flexural areas, nape of the neck, and dorsum of the feet and hands. In adult patients, lichenified/exudative flexural dermatitis is common alone or with head, neck, and hand eczema, and (or) prurigo nodularis ^[3][4][3,4]. Patients with AD often experience itching, sleep disturbance, and abnormality in social, mental, and emotional function [5]. In particular, the chronic relapsing course of AD poses a significant negative effect on the quality of life of the patients and (or) their family caregivers ^[6][7][8][6,7,8]. AD is often concomitant with other atopic diseases, such as asthma, allergic rhinitis, and food allergies ^[9][10][9,10]. It can also increase the risk of other non-allergic comorbidities, such as many autoimmune- or immune-mediated diseases and mental disorders [11]. AD has become a global health issue since it causes high health-care costs worldwide and is associated with considerable morbidity and impairment of quality of life [12].

The prevalence of AD may vary by race: in the United States, the prevalence of AD in children is lower among whites (11%) than among African Americans (17%) ^[13][14][13,14]; the prevalence and persistence of AD in certain non-white racial/ethnic subgroups are higher than in non-Hispanic whites [13]. Studies have suggested that the incidence of AD is associated with industrialization. Over the past few decades, the overall prevalence of AD has increased two- to three-fold in industrialized countries, especially in Europe, the United States, and Japan, with the highest prevalence approaching 30% in some populations ^[15][16][17][15,16,17].

In China, dermatologists have made continuous efforts on research of AD in a Chinese context, from adapting the definition of AD to include eczema ^[18][19][18,19] to engaging in both clinical and basic research to cover the topics of AD relating to genetics and epigenetics ^[20][21][20,21], clinical and molecular phenotypes [22], immune response ^[23][24][25][23,24,25], the environment [26], and microbiomes ^[27][28][29][27,28,29]. 

Among children, the prevalence of AD ranges from 10% to 20% in developed countries ^[30][32]. During the past two decades, the prevalence of AD in China has been increasing greatly, approaching those of Western countries and developed Asian countries ^[31][32][33][33,34,35], although the increase is later than those of Japan ^[34][36] and Korea ^[35][37]. The prevalence of AD diagnosed using the United Kingdom (UK) working party criteria was 0.69% in adolescents (aged 6–20 years) in China in 1994 ^[36][38]. One study in Shanghai (China) district reported an prevalence of 8.3% in children aged 3–6 years in 2012 [26]. A study including twelve metropolises in China showed a prevalence of 12.94% in children aged 1–7 years in 2014, and the proportion of mild AD, moderate AD, and severe AD was 74.60%, 23.96%, and 1.44%, respectively. A more recent study reported that the prevalence of AD in infancy was 30.48% according to clinical diagnoses of dermatologists in China [19]. According to epidemiology data, the prevalence of AD in China is decreasing with age, which is also lower in rural areas. Passive smoking, premature birth, and choosy in food are risk factors for AD ^[31][33]. From 1990 to 2019, the number of patients with AD was increased by 25.65%, 3.85% of which was due to population aging, 20.16% due to population growth, and 1.64% due to age-specific prevalence ^[37][39]. The number of patients with AD was about 35.58 million in China in 2019 ^[37][39]. Childhood lifetime-ever eczema prevalence ranged from 10.0% to 30.0%; in total, 15.5 million children aged 3 to 6 years old and 70 million adolescent/adults aged 15 to 86 years old had lifetime-ever eczema in mainland China ^[38][40]. The prevalence of AD in China might be lower than that of Caucasians. A systematic review by Bylund et al. reported that the 1-year prevalence of doctor-diagnosed AD ranged from 1.2% in Asia to 17.1% in Europe in adults ^[39][41]. In a worldwide comparison, Swedish children showed the highest level of AD incidence (34%), and the lowest level was observed in Tunisian children (0.65%) ^[40][42].

As shown by the Global Burden of Disease Study, skin diseases are among the leading causes of the global burden of non-fatal diseases ^[41][43]. Compared to the general population, patients with AD report impaired health-related quality of life, accompanied by increased levels of itching, pain, sleep disturbances, anxiety, depression, and lower work productivity ^[42][43][44,45]. In younger patients, AD also has negative effects on the quality of life of their parents. Descriptions of the current status of disease burden of AD and its future trend would help to determine the direction and focus of public health, and provide policy-makers with data-based information to allocate health care resources accurately and effectively ^[44][46].

AD is a lifelong disease with a recurrence rate up to 75.9% within 7 years ^[45][47], placing huge and persistent economic and mental burdens on the patients and their families, and AD ranks first in the burden of non-fatal skin diseases ^[44][46]. Years lived with disability (YLD) reflects not only the incidence of the disease, but also the degree of disability caused by the disease. As a result, it gives a better indication of the severity of the epidemic. AD is a chronic disease with a big economic impact and affects both children and adults, potentially impairing productivity. In the case of non-fatal diseases, YLD is more suitable for estimating the burden of AD ^[46][47][48,49]. According to the Global Burden of Disease Study 2019 (GBD 2019), there were 171.17 million individuals and 7.48 million YLDs due to AD around the world in 2019. Among a total of 369 diseases, the percentage of YLDs due to AD was 0.87%, ranking twenty-eighth ^[37][39]. In China, among the 369 diseases, the age-standardized YLD rate of AD ranked twenty-fourth. From 1990 to 2019, the number of AD patients and YLD in China increased rapidly (1.43%) with population growth and aging.

The burden of disease can also be expressed in terms of disability-adjusted life years (DALYs), a measure of the difference between living in perfect health and living with an illness. DALYs are calculated as years lost due to disability or its consequences, plus the years of life lost due to premature mortality ^[48][50]. In non-fatal diseases such as AD, DALYs are measured primarily in the number of years lost to disability. The global DALY rate for AD was 121 in 1990 and 123 in 2017. China was among the five countries with the lowest age-standardized DALY rates (82.1, 44.2–138) due to AD in 2017 ^[44][46]. Generally, higher AD severity is associated with greater burdens on work productivity. Data reported by Andersen et al. show that patients with mild AD, moderate AD, and severe AD lose 2.4 h, 9.6 h, and 19.0 h per week, respectively ^[49][51]. Thus, AD improvement would reduce the economic burden.