others at 3.6%, 1.9%, 1.6% and 1.4%, respectively (46). 1 Incidence and prevalence are related terms. Prevalence refers to the proportion in a population found to have a condition out of the total number of people studied, while incidence refers to new cases within a specified period (usually one year) divided by the size of the population. Global report on psoriasis 8 Table 1. Summary of studies on the prevalence of psoriasis Geographic location Year(s) of the study Sample size Estimate Female Male Ages of subjects (years) Children China, Taiwan (39) 2004 4067 0.00% 6–11 China, 5.1% all ages Note: ... = no data. Psoriasis is considered equally prevalent in both sexes (5). However, out of all studies that reported prevalence by sex (Figure 2), some studies indicated that psoriasis is more common in men. Quoted values are not statistically significant. This issue needs further investigation, in particular, the differentiation of genetic and behavioural factors. Figure 2. Prevalence of psoriasis by sex (year of data survey given; not of publication) Denmark 1978 Male Female Prevalence Germany 2005 Norway 1979–Global report on psoriasis 10 Psoriasis can occur at any age. While some studies indicated the average age of onset for psoriasis was 33 years of age, and 75% of cases occurred before 46 years of age (93), others suggested that the onset of psoriasis was bimodal with two peaks of the disease – the first between 16 and 22 and the second between 57 and 60 years of age (100). Psoriasis also occurs in children. However, there are few studies on the incidence or prevalence of psoriasis in children, and those that do exist reveal variations between almost absence of juvenile psoriasis in Taiwan, China (38,39) and 1.37% lifetime prevalence in 0–17-year-old children in Germany (52). The largest study on prevalence among children was carried out in Germany in 2007 (49). Data collected from a health insurance company database of about 1.3 million individuals showed the prevalence of psoriasis in children younger than 18 years of age was 0.40%, and increased roughly linearly over the life course. In 2008–2009, a study of 2194 children in Egypt (47) found that the prevalence of psoriasis among people 18 years of age and younger was 0.05%. Is psoriasis becoming more or less common? Tracking trends in incidence and prevalence of psoriasis is difficult, due to the different methodologies of research on this issue. However, an apparent upward trend is observed in several countries. The prevalence of psoriasis in China in 1984 was 0.17%, while 25 years later, another study found it to be 0.59% (37,43). The prevalence in Spain in 1998 was 1.43%, while 15 years later it was reported as 2.31% (86,87). Data on the prevalence in the United States from the National Health and Nutrition Examination Survey indicated an increase in prevalence from 1.62% to 3.10% from 2004 to 2010 (46,69). However, different methodologies of conducting these studies, especially in view of the various case definitions of psoriasis, do not allow for a clear assessment of the increasing trend. The most instructive study of trends in prevalence is a 30-year follow-up of a population-based cohort in Tromsø, Norway (3). Analyses of trends in prevalence during 1979–2008 indicate that selfreported lifetime prevalence of psoriasis (in the same cohort) has increased from 4.8% to 11.4%. Global disability burden from psoriasis The Global Burden of Disease Study in 2010 (1) was an attempt at measuring the extent of disability or loss of health due to different diseases. One of the metrics commonly used for this measurement is the disability-adjusted life year (DALY). DALY equals the sum of years lived with disability (YLDs) and years of life lost (YLLs). One DALY equals one lost year of a healthy life. The analyses of the Global Burden of Disease Study suggested that the burden of psoriasis is high (1). Global average DALY for psoriasis for 2010 was estimated at 1 050 660, which is twice as much as for acute hepatitis C. The global burden of disease estimates for individual countries are presented on the map in Figure 3. DALY = YLD + YLL 1 DALY = 1 lost year of healthy life Global report on psoriasis 11 Figure 3. World map showing the DALYs for