Demographic and Epidemiological Transitions
As Bob Dylan sang, “The times they are a-changing.” As the cliché goes, change is the only thing that is unchangeable in this world. It is true that even in terms of public health and global health, we see change almost constantly. But there are certain driving forces which are responsible for the direction as well as the dimensions of the change. And it’s important that we understand them so that we can not only predict what is likely to happen, but also position our public health interventions so that we anticipate and avert some of the major public health problems as well as advance health equity in the desired direction.
The transitions that shape public health have been described by Rayner and Lang as demographic, epidemiologic, urban, and nutritional. These are traditionally seen by most public health experts as the important drivers of health transition. However Rayner and Lang also emphasize that there are others that we do not conventionally take into account but nevertheless are very important for understanding changes in public health. The economic transition, energy transition, the biological and ecological transition, cultural transition and democratic transition. Of these, the demographic transition is the most widely studied and most often discussed. This is where there is a transition within a country from high birth and death rates to low birth and death rates, along with the development from a pre-industrial to an industrialized economy. Warren Thompson developed this concept in 1929. And then he basically based it on the historical demographic observations in developed countries and later on this has been adapted to the study of developing countries as they are in developmental transition with accompanying epidemiologic and health transition.
In terms of demographic transition we are looking at five stages where in the earliest stage, which is high stationary, we find a fairly high death rate as well as a high birth rate on the population. And when we look at the population itself, because the deaths and births are canceling themselves out, the population growth is actually high stationary. Then we see a stage where the death rates start falling first and the birth rates continue to be high and therefore the population starts expanding. This is early expanding.
In the later stage we see the birth rate also beginning to fall but the death rate continues to fall fairly sharply and therefore we see a late expanding stage of the population. Then by the time the death rates and the birth rates both fall to low levels, where they reach approximately each other, the population growth becomes stationary and the population level remains stable. But then the birth rate continues to decline as in some advanced countries like Germany and Italy and there, because the death rates have now fallen to a lower level, but the birth rates are fallen even to a further lower level, the population starts declining. And that is where we find the fifth stage in some countries.
So there are several countries which are going through these phases and demographic transition therefore is an important concept in understanding how the age profile of the population, the size of the population, as well as the disease patterns of the population are determined. When we look at China for example, we see a huge shift in the demographic profile from a pyramidal shape in 1990 to a virtually, a cylindrical shape in 2050, where we see that the bulge is mostly in the middle and the top of the age profile, rather than at the bottom of the age profile where the young predominate. And we see also this happening in terms of life expectancy. As we understood previously from the millennium Preston curve, as per capital income rises, we find life expectancy rising up to a certain level and then gradually plateauing it off. And we found this in the case of China, where up to 1970, life expectancy rose very sharply as the economic development went in and also the social determinants like water, sanitation, nutrition were addressed in a very equitable manner across that society.
But then the population growth started stabilizing a bit and life expectancy also started plateauing off a bit. On the other hand, in India we are seeing a steady rise in life expectancy as we move from 1960 to 2010. And as a result of this, what we see is thatthe total fertility rate in China has fallen over a period of time. This of course in China has been accelerated by the one child policy. On the other hand, in India, we do see a falling fertility rate, but far less sharply than in China. As a result we see also a relatively delayed demographic transition in India in comparison with China. In China, the working age population actually peaked close to 2010 and is now on a decline in terms of the ratio of the working to the non-working age population. So China has already had its demographic window of opportunity when there is a large expansion of the working age population, which can give a thrust to economic development through increased productivity. India is still to have that demographic window of opportunity in terms of a rising working age population. This is beginning to happen now and we can anticipate an accelerated economic growth from this demographic dividend.
However, in order to harness this demographic dividend, India will have to invest both in education and skills for the young people, but also in protecting the health of the population. That is where public health becomes a very important element for economic development. Otherwise the demographic dividend can turn into a demographic disaster with a large number of working age people suffering from early death or prolonged disability.
The concept of epidemiological transition based on demographic change was originally propounded by Omran who described three stages. In the first stage where societies still experience a lot of pestilence and famine, the average life expectancy is about 35 years. And infectious diseases and nutritional deficiencies dominated that society. And later on, as people moved from a life expectancy of about 35 years to 50 years, the pandemic started receding and we started seeing the slow advent of non-communicable diseases like hypertension. But even those were substantially related to hypertension with hemorrhagic bleeding stroke. And we found that later on, in the mid-20th Century we saw the age of degenerative disease becoming the dominant epidemiological profile of a demographic transition. And in this, life expectancy rose to about 60 or above, but we found that by now the clotting stroke and ischemic heart disease or coronary heart disease became the dominant killers, infections receded, particularly major infections which killed people all across the world, became, less of a public health challenge. And even nutritional deficiencies were substantially corrected in several parts of the world. But this was a profile seen mostly in high income countries. Low income countries continued to experience some of the earlier phases of transition. And then we also had other transitions that were added on later in which we had a stage of delayed degenerative disease which is proposed by Olshansky and Ault, who said that by the time United States and Europe crossed the 1980s, we started seeing life expectancy cross 70 years, people still died dominantly of non-communicable diseases like ischemic heart disease, stroke and cancers, but many of them died above the age of 70 years. What we saw in Russia was a reversal after the fall of the Soviet Union where a stage of social upheaval suddenly came in in a period of transition life expectancy among Russian males fell to 58 years the lowest in the century because of the variety of reasons cardiovascular death rates went up, alcoholism became a major problem killing a larger number of people, and that actually can have a reversal of the epidemiological transition. But we can also anticipate that there could be an era of environmental degradation if we do not protect the environment on the planet.
And we do not know exactly what’s going to happen in terms of resurgence a of a large number of infectious diseases added on to the already existing problem of non-communicable diseases and how that is going to play out. We should try and avert that by protecting the environment. So these are the kind of transitions that we really ought to be looking at in terms of various developing countries and seeing the stages in which they’re transiting. But one of the important lessons is we don’t necessarily have to go through each phase for exactly the same length of time that the high income countries experienced. By utilizing the knowledge that we already have, we should try and telescope the transition by abbreviating the stage 3, where there are large number of mid-life deaths and move quickly onto stage 4, where even if people die of non-communicable diseases, they’ll die mostly after the age of 70 years.
This particular health transition model has been very helpful in providing an model for predicting what’s going to happen and understanding some of the dynamics. However, it’s been somewhat limited because it is focusing mainly on proportional mortality. While giving us an evolutionary perspective rather than a limited cross-sectional view, it is underestimating the burden of non-communicable diseases in countries of Africa for example where we see in Tanzania already the age standardized mortality rates of stroke in the age group of 15 to 59 far exceed those in England and Wales. But you don’t capture that by only looking at proportional mortality, because HIVAIDS, malaria, and others are actually killing a large number, therefore the proportional mortality due to non-communicable diseases appears to be lower. So the model that was proposed by Omran which is mainly based on proportional mortality, looking at the ratio of deaths from a particular cost to total mortality serves a useful purpose, but has some limitations. The other limitation is, health transition is not simply a linear model, because all of these are likely to be determined by complex interactions among various determinants. And these complex systems are nonlinear. So while we do depend upon the health transition model, the epidemiological transition model, we will have to look at other elements which determine global health. Some of these transitions can occur very rapidly. For example in Mexico we have seen between 1950 and even by year 2000, a massive increase in the deaths attributable to non-communicable diseases and a substantial decrease in the deaths attributable to infectious diseases.
So transitions are happening very rapidly because unlike the leisurely pace of transition that took place in Europe and America in the 19th and 20th Century, we are seeing the forces of rapid urbanization and globalization propelling countries in the low and middle income group into a rapid health transition with a very speedy rise of non-communicable diseases. And sometimes the coexisting burdens of infectious diseases and non-communicable diseases overwhelming the health system simultaneously as in the case of India.
Now in terms of nutritional transition. The traditional diets are now being replaced by western diet patterns. And there is an accelerating rate of change in diet, particularly through transnational trade, aggressive marketing of unhealthy food products. Simultaneously, there is a shift in physical activity patterns with a large number of labor saving devices at home and for transport. We now find that people are far less physically active. In fact this is the paradox of modernity that previously people used to be paid for doing physical work, now people have to pay for doing physical work. They have to actually become members of a gym. So we have seen a fair amount of changes in the nutrition profile across the world. But this pace is not uniform across the world.
For example in Asia, the number of undernourished people are declining as demographic and economic transitions take place. But in sub-Saharan Africa, there still continues to be a huge unresolved public health challenge. In terms of obesity however, we are seeing a rapid rise almost in every region of the world. And we are seeing that overweight and obesity are going to be one of the major public health challenges of the 21st Century, not only because they’re a problem by themselves, but they set the stage for a huge rise in non-communicable diseases like cardiovascular diseases and diabetes and cancers.
There are other transitions also that we must understand. The biological and ecological transitions. When we start deforesting and herding animals close together for increasing our meat consumption, we are also creating a conveyor-belt for vectors and viruses to migrate from wildlife and forestry into veterinary population and human habitat. As we alter the environment and create climate change that is also going to affect our health in very many ways. So understanding ecological determinants of health is becoming even more important in the 21st Century than ever before. Even in terms of energy security, this becomes very important. Because the pursuit of cheap energy is also shaped by the desire to replace human and animal labor. But as we move towards increased energy utilization we also have to see what it is doing to the environment, ambient air pollution, indoor air pollution, and also to see whether that is also transforming our life in terms of reduced physical activity.
All of these matter. And even energy security across the world has economic implications which in turn determines how health systems are funded. So all of these are going to be very important, because there are countries which provide huge energy subsidies. Can some of that be withdrawn by reducing the amount of energy dependency we have and the savings be transferred to health for universal health coverage? These are the kind of questions we ought to be discussing in global health in the 21st Century.
Similarly, democratic transitions are also very important. Because how much of decisions are made in a country by the people at various levels? Are communities empowered? Are the provinces and regions empowered? Or is everything so centralized and dictated by a central authority with very little participation of people is going to be important. In the 21st Century democracy is moving from a representational model to a participatory model. Social media are becoming very important. And therefore a lot of public health knowledge is going to be disseminated and a lot of public health action is going to be driven by this kind of participatory democracy. So these are all trends that are going to be determining global health in the 21st Century. So when we look at all the determinants, we look at demographic shifts such as aging, urbanization and industrialization which alter living habits, globalization, which actually through marketing can accelerate a lot of changes in the way we live and education and culture which shape our beliefs, not in one community but across the global community and poverty which limits access to health both in terms of being a barrier for health information as well as health services.
And we are looking at urbanizing environments where the built environment can either be a barrier to physical activity or it can be an enabler or it can create pollution or provide clean air. And underlying all of these, there are also vectors like tobacco, unhealthy food and alcohol which are rapidly driving the health transition across the world and towards obesity and non-communicable diseases. Understanding these determinants is absolutely pivotal to global health in terms of its actions in the 21st Century.
So we understand health transition, but we need to respond to it. There are populations because of demographic and social determinants which are being propelled from low risk to high risk. Like rural populations, populations in low and middle income countries which are moving to westernized patterns of living in terms of food consumption, or tobacco consumption.
There are also.people in each country who are actually at low risk. Children, children are not born with a high risk. But the way we actually nurture them in our society can create high risk for them. So we need to provide these low risk populations from moving to high risk through public health interventions which create a healthy society.
At the same time in every population, there are individuals who’ve already acquired high risk. We need to return them to low risk by clinical and behavioral interventions. So while high risk may be determined by a combination of biology, beliefs and behaviours, there are a number of interventions that we have which can actually reduce the risk. And our public health interventions in the 21st Century must combine all of this knowledge into effective interventions both at the population level and at the individual level through competent health systems which take the lessons of health transition into account while framing global health policies and shaping global health practice.