Social determinants of acute respiratory infection: A systematic review

Research has shown a link between low socio-economic status (SES) and higher acute respiratory infection (ARI) rates, but the mechanisms leading to these inequalities remain unknown. Income and education have been shown to be related to ARI risk, and race/ethnicity and indigenous identity were linked to disease and mortality rates during the 2009 H1N1 influenza pandemic. Area-level factors are also related to ARI inequalities: hospitalization rates were significantly higher in high-poverty than in low-poverty areas in the United States during the 2009 pandemic and area-level deprivation indices have been linked to worse ARI outcomes in the UK. Differential exposure, susceptibility to disease, and access to healthcare have been proposed as mechanisms that could generate ARI inequalities. Differential exposure due to population structure—differential household size, population density, and age-structure—could lead to unequal ARI rates.  Once exposed, differential susceptibility due to differential vaccination, chronic condition prevalence, and smoking rates could further impact incidence rate inequalities. Differential delays in healthcare access and rates of underlying chronic conditions could result in unequal hospitalization rates but also in differential exposure to those with more severe disease. The literature linking these behavioral and biological differences to ARI inequalities has not been systematically reviewed. We conducted a systematic review of the literature using PubMed and Scopus databases. Studies that examined the relationship between SES (income, poverty, education status, healthcare indigence, occupation, and race/ethnicity) and ARI incidence or severity were included. We examined 5559 titles and abstracts, and excluded 4877 studies because they did not fit our inclusion criteria. The full texts of 682 studies were reviewed, and studies that fit our criteria were summarized in a narrative review. We documented the population of focus (developed or developing country), and whether the study found a direct, inverse, or no relationship between SES and ARI. For studies that found that low SES was related to worse ARI outcomes, we documented whether they proposed and tested a mechanism, and if yes, whether the mechanism explained the relationship fully, partially, or not at all. Two-thirds of the included studies focused on a developed country; close to 90% found that low SES was linked to poorer ARI outcomes, and half of these proposed and tested a mechanism. Differential rates of underlying chronic conditions, crowding, vaccination, second-hand smoke exposure, breastfeeding, and smoking were mechanisms examined in multiple studies. Differential cooking-fuel quality, and obesity and malnourishment rates were proposed in fewer studies. Other mechanisms examined included differential access to clean water, healthcare, timeliness of healthcare access, and cross-ventilation in the home. We will use our findings to present a causal diagram of the link between SES and ARI inequalities highlighting both, the potential mechanistic causes as well as gaps in the literature. The study lays the foundation for viewing ARI inequalities through a systems thinking lens. We will discuss the importance of a systems thinking framework to understanding the causes of and planning interventions to reduce persistent ARI inequalities.