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Social cross-over in cardiovascular disease: context matters

The concept of the ‘social crossover’ in cardiovascular disease (CVD) mortality was first described in 1978 by Marmot et al. By analysing mortality trends between 1931 and 1971 in England and Wales, Marmot et al found that the pattern of mortality from coronary heart disease changed over the years, from being more common among middleclass and upperclass men and women to being more common among workingclass men and women. Underlying this change was a steep increase in CVD mortality among the workingclass group starting in the 1950s for men and in the 1960s for women, concurrent with increases in smoking rates and dietary changes. In the study by Mallinson and colleagues, the authors describe the absence of a social crossover in CVD mortality in Brazil. The study used crosssectional data for 2010 from states at various degrees of economic development to proxy the changes in economic development experienced by highincome countries over time. Mallinson et al found no support for the social crossover hypothesis. Instead, they found that educationbased inequalities in CVD mortality were actually widest in the least developed states, meaning that CVD mortality was highest among the population with the lowest levels of educational attainment. Previous studies on the social crossover have relied on longitudinal samples to examine how inequalities in CVD change over time. Mallinson et al instead leverage intracountry heterogeneity in social development. The use of crosssectional data from one country has two important advantages: (1) uniformity of the vital statistics system across the country, which reduces inconsistencies in the quality of the data often present in crosscountry comparisons, and (2) consistency of coding, one of the biggest issues in studies using historical data. The authors also addressed issues of variation in the quality of data across states and examined the impact of potential measurement biases in the results. However, using crosssectional data comes with its own problems. In this piece, we will discuss the results from Mallinson et al in the context of changing patterns of CVD mortality in lowincome and middleincome countries (LMICs) and their implications on clinical care and population health in LMICs. The original social crossover paper by Marmot et al used data for a historical period marked by consistent increases in CVD mortality, following a previous transition from high mortality due to infectious diseases to high mortality due to noncommunicable diseases. This transition, also experienced by other highincome countries, was followed by rapid declines in CVD mortality, leading to a subsequent continued (although slowed, compared with previous decades) growth in life expectancy. In addition to the overall decline in CVD mortality, highincome countries also presented a progressively strong social gradient in the distribution of CVD mortality in the 1970s and 1980s, with higher rates of CVD mortality among socially disadvantaged groups compared with advantaged groups. In summary, the phenomenon described as social crossover of CVD mortality occurred in a specific historical context in highincome countries. However, the patterns of mortality observed in highincome countries do not seem to be a good proxy to explain present patterns of CVD mortality and CVD inequalities in LMICs, which currently account for three in four CVD deaths worldwide. The idea of a clear transition from high burden of communicable diseases to high burden of noncommunicable diseases is unlikely to be seen in LMICs. Many LMICs, including Brazil, have experienced demographic changes such as rapid urbanisation and population ageing, which contributed to a higher burden of noncommunicable diseases. This higher burden has occurred in (1) populations in a different baseline status, as compared with highincome countries, including high levels of malnutrition and stunting, creating a double burden of disease, and (2) a historical context marked by the globalisation of economic activity and a consistent increase in the availability of cheap, poorly nutritious calories. In Brazil specifically, ischaemic heart disease and stroke have been the leading cause of death for the past 50 years. At the same time, income inequality and poor infrastructure in large cities have allowed the persistence of communicable, maternal, neonatal and nutritional disease despite expansion of primary care, improvements in vaccine coverage and reductions in infant mortality thorough the country. These improvements have occurred concurrently with the increased availability of ultraprocessed foods through multinational food corporations.