Global trends in eczema prevalence during early life

It is often stated in public and academic discourse that we are living in the midst of an allergy ‘epidemic’ or public health crisis. Atopic diseases such as eczema, asthma, hayfever and food allergy do appear to be more common in certain regions, especially high-income, urbanized settings. There is significant variation in prevalence worldwide, and while awareness of allergic diseases, allergy service provision, testing, pharmaceutical and device treatments are all increasing, hard evidence for ongoing increases in disease burden is difficult to generate.1, 2 This is partly because studies have often used different methodologies of disease diagnosis (e.g. questionnaire-derived data vs. physical examination), hampering direct comparisons. One of the best, and certainly the largest, international surveys of allergy epidemiology in recent decades has been the International Study of Asthma and Allergies in Childhood (ISAAC). ISAAC Phases I and III used validated questionnaires to assess the prevalence of asthma, eczema and allergic rhinitis in around half a million school-age children across over 50 centres. At the time of the ISAAC surveys in the 1990 s, these were some of the largest epidemiological studies ever undertaken. ISAAC showed allergic diseases to be common, but documented wide variation in prevalence internationally and even within countries.3 ISAAC used the same methodologies in the same populations 5–7 years later to look at trends in disease prevalence, showing an increase in most centres that had a low prevalence in the first survey, while there appeared to be a plateauing or even decreases in hitherto high prevalence regions, such as the UK and Scandinavia.4 In this month's issue, Langan et al build on the ISAAC I and III surveys, repeating the exercise once more, in a subset of ISAAC centres.5 This formed part of the Global Asthma Network (GAN) Phase I study, and involved over 100,000 school-age children from 27 centres in 14 countries. Children and their parents were cross-sectionally surveyed in 2015–2020 and the same questionnaires were used as in the previous ISAAC surveys, with response rates of 90% for adolescents and 79% for children, meaning that the data are likely to be representative of the populations studied. GAN data were combined with ISAAC I and III data from the same centres to elicit global trends in eczema prevalence between 1993 and 2020. Findings were consistent with a slow, ongoing increase in eczema prevalence, with ~1% increase in prevalence per decade since ISAAC I in 13–14 year olds, and a similar rate of increase in 6–7 year olds. Overall, ~6% of children in both age groups were reported to have current eczema symptoms. The most notable finding of this study, however, was the high variation in prevalence trends over time. Eczema does not seem to be consistently increasing in low income countries or in the Americas, and has increased in some areas of Africa and the Eastern Mediterranean as well as higher income settings. The marked variation in eczema prevalence has been noted since the first ISAAC survey and remains largely unexplained. However, questionnaire-derived prevalence data may not be the optimal approach for establishing risk factors.6 To drill down further into risk factors driving the observed differences in eczema burden between countries will require more in-depth studies of patient populations, ideally examining patients physically for eczema phenotypes and disease severity, also integrating skin microbiome and immune profiles as well as environmental exposure data, collected at the level of the individual patient. This is now feasible through mobile App and Artificial Intelligence technology. In addition, there remain a lot of geographical gaps in the eczema burden data, in particular in rural areas and many low income settings worldwide. The latest GAN survey data reconfirm the significant global burden of eczema among children and adolescents and will form the basis for future studies to understand the underlying genetic, immunological and environmental disease drivers to help us develop effective methods of disease prevention for this common disease (Figure 1). The first article in our virtual special collection on Allergy in India is published in this issue also, and this documents the prevalence of allergic diseases in India.7 While asthma and allergic rhinitis are common in India, rates of eczema and food allergy are remarkably low—some of the lowest in the world—suggesting powerful protective factors which are important to understand in relation to the prevention of these conditions in other global regions. There will be much more published about allergy in India in this journal in the coming months, as part of our special collection on the topic. This month's Cochrane Corner summarizes evidence for pulmonary rehabilitation for treating adults with asthma.8 The authors found that this non-drug approach to treating asthma improves quality of life and functional exercise capacity. It is still uncertain, however, whether pulmonary rehabilitation has a direct effect on asthma pathophysiology and thereby on asthma-specific outcomes such as exacerbations or asthma control. Further work is clearly required to understand whether pulmonary rehabilitation and other non-pharmaceutical approaches can impact on disease-specific outcomes in asthma, but the intervention certainly appears unlikely to harm patients and is likely to improve overall well-being (Figure 2).