Using standardized WHS data obtained from the WHO, we estimated that the global prevalence of clinical asthma in adults was 4.5% and varied by as much as 21-fold amongst 70 participating countries. Amongst the population living with clinical asthma, almost one in four was a current smoker, one in two reported wheezing in the past 12 months, but one in five had never received asthma treatment. The WHS is the first standardized, representative survey which included population-based data regarding respiratory symptoms and treatment permitting estimation of the global burden of asthma in adults. This study provides the most current global estimates of the burden of asthma and shows that asthma continues to be a major public health problem worldwide.
We estimated the prevalence of asthma using three definitions ranging from the most stringent - self-reported doctor diagnosis, to the most inclusive definition - self-reported wheezing. Clinical history in combination with a reversible airway obstruction as measured by a pulmonary function test is the gold standard for diagnosing asthma. However, implementing such a standard to identify individuals with asthma is impractical considering the scale of the sample of this study, and would be extremely costly and time consuming. Our questionnaire-based classifications offer a reasonable, feasible and practical alternative. While the respiratory symptoms definition may overestimate the global asthma prevalence, the clinical definition likely underestimates disease burden in resource-poor countries with inadequate access to health care facilities and treatments. Our analyses focused mainly on the clinical asthma definition, which identifies asthma burden based on diagnosis and/or treatment. This definition likely yields a lower false positive rate compared to a symptom-based definition [17
Compared with the asthma estimates previously reported (Table ), our asthma estimates are more up-to-date, are based on consistent data contributed by a large number of developing countries, and include estimates for both rural and urban dwellers. Country and regional differences highlight the need for locally tailored interventions and initiatives to address the specific risk factors and needs. While not directly comparable due to differences in methods used, our country-specific estimates are broadly similar to those presented in the GINA report [4
]. We also observed that the prevalence of asthma varied greatly between countries, with the highest prevalence observed in resource-rich countries [4
]. The ISAAC study also employed a standardized global survey and was implemented locally, but the target population was children only. Unlike the WHS, some countries in the ISAAC study used convenience samples and were not necessarily multi-staged or stratified to be representative of the entire country. The ISAAC study used a video questionnaire to help reduce misclassification due to translation of wheezing symptoms; this, however, was not practical for the WHS. The ECRHS was limited to developed countries, and therefore cannot be used to infer global figures. Finally, since the GINA Burden of Asthma 2004 estimates were a retrospective combination of the ISAAC and ECHRS surveys, the country specific estimates are not necessarily representative of the entire population, and the averaging to different surveys from the same country introduces bias since different instruments were used.
Summary of prevalence of asthma reported in the literature
In 2010, Sembajwe et al. used the WHS data, and reported variations in wheezing symptoms and doctor diagnosed asthma prevalence across world regions relating them to national income [13
]. All subjects aged 18 to 99 years from 64 countries were included in their study. They reported a 6% prevalence of doctor diagnosed asthma and 9.2% for current wheezing, which does not agree with our findings. The major differences between their findings and ours may be attributed to differences in study population included (ours included all 70 participating countries but limited it to participants aged 18 to 45 years old). Since our prevalence estimates were lower, it suggests that their estimates may have been biased by the inclusion of subjects with COPD as asthma.
Using the WHS data to measure the global burden of asthma offers several strengths. Firstly, the same standardized questionnaires were applied to all individuals who participated. Secondly, the survey was administered using multi-staged random sampling in most of the sites making the country-specific estimates representative of the whole population. The survey in each country was also stratified by age, sex and rural/urban residence, further improving the generalizability of our findings. Nevertheless, the WHS data only included adults, and likely underestimates the global burden, since asthma is more prevalent in children. Canada and the United States are notable absences from the Survey, however participation was voluntary and these countries elected not to take part. Therefore, our estimates likely underestimate the total global burden of asthma, but the sample of countries included in the WHS is sufficient to make statistically sound global inferences
Our results highlight that asthma continues to be a major public health concern worldwide. Applying our 4.5% clinical asthma prevalence to the current world population of 7 billion translates to 315 million individuals with asthma. However, using our 8.6% self-reported prevalence of asthma symptoms, we estimated that nearly 623 million individuals are currently living with some level of asthma-related symptoms worldwide. While proper long-term management of asthma will allow individuals with asthma to achieve good levels of control enabling them to live with good quality of life, our data indicates that asthma control is not optimal in many countries. Worldwide, nearly half of the asthma population reported wheezing in the last 12 months, and only a moderate proportion had been diagnosed and/or received treatment. In addition, the high prevalence of smoking continues to be one of the major barriers in combating the global burden of asthma. While the highest overall prevalence of asthma was observed in resource-rich countries, many resource-poor nations also have a high prevalence of this disease. This is of concern because in most such countries, resources are consumed by the pressing demands of infectious diseases and the need to provide primary care for the broader population. In many countries there is little, if any provision of the essential medications that at both individual and population level can lead to very satisfactory control of asthma. Uncontrolled asthma poses an extra weight in the burden of non-communicable disease, which constitutes a major barrier for development.