The epidemiology of acute respiratory infections in children and adults: a global perspective.

While a number of advances have been made in our understanding of the epidemiology of acute respiratory infections in the past two decades, a number of serious questions still require urgent answers. The associations of factors such as chronic disease in adults, direct smoking, passive smoking, crowding, and breast feeding to acute respiratory infections are now well documented. Appropriate changes in public health policy need not be predicated on results from still further studies. However, in virtually all of the other areas cited in this review, further data are required. In developing countries, studies being currently conducted on vitamin A supplementation, malnutrition, and indoor air pollution will help address the most pressing issues. More studies are also needed on the relations between HIV infection and acute respiratory infections, as well as low birth weight and respiratory infection. The National Research Council studies have provided important additional data on etiologic agents in children in developing countries, but data on adult pneumonia remain sparse. In developed countries the issues that may be of greatest interest are the relation between maternal antibody levels and passive immunity in infants, the reasons for the increase in pneumonia mortality in older age groups, and the relation between air pollution and acute respiratory infections (as opposed to morbidity from bronchial reactivity). From a methodological viewpoint, the relation between previous respiratory infection (particularly in the first year of life) and subsequent acute respiratory infection morbidity has been inadequately explored. Adjustment for autocorrelation in multivariate models may be necessary if this relation is strong. Greater standardization of data collection methods in developed and developing countries also needs to be more seriously addressed. Given that some advances have been made in this area, the time may be right for development of acute symptom questionnaires, akin to the American Thoracic Society chronic respiratory questionnaire, for use in both developed and developing countries. Standardization of diaries, although somewhat more difficult, would also be extremely useful in many instances.