Demographic and epidemiological transitions

DAVID ROSS: We’re now going to talk about demographic and epidemiological transitions and their importance for adolescent health. Adolescents represent one sixth of the world’s population. In 2014, there were just over 1.2 billion adolescents in the world, the most that there’s ever been, and that’s estimated to increase very slowly to about 1.23 billion by 2040, after which the number will decline. The trends in population around the world, in different regions of the world differ substantially. As one can see here, it’s projected that there’ll be a substantial decline in the number of adolescents in the East Asia on the Pacific over the next 35 years. Whereas, the numbers in Sub-Saharan Africa will continue to increase substantially through to the 2050 and beyond.

Out of the total population, the proportion who are adolescents vary substantially by region, with the greatest proportion in the least developed countries and the smallest proportions in the industrialised countries. So what is the demographic transition? This is caused by rapidly falling fertility rates and huge improvements in child survival. Here we can see the total fertility rates. That’s the number of births per woman in selected countries for over a 30 year period from 1980 to 2010. And one can see here that in all of the low and middle income countries, that’s Brazil, China, India, Nigeria, the number of births per woman has been declining substantially between 1980 and 2010.

Whereas, in the United Kingdom, an example of a high income country, there’s been very little change over that same period, and it’s already– it was already very low to start with. In addition to the decline in fertility rates, there’s also declines in age specific mortality rates. Here, looking at this, this is data from 50 countries between 1955 and 2004 on their mortality rates at different ages. The purple line which starts as the highest line in 1955, you can see has declined substantially over that period. Those are the death rates in one to four-year-olds. In this case, in males. And you can see the same general pattern in females.

On the other hand, the mortality rates in 10 to 14-year-olds, 15 to 19-year-olds, and 20 to 24-year-olds have not been declining at the same rate so that in fact, mortality rates are now considerably higher among 20 to 24-year-olds than among one to four-year-olds in high income countries and the higher and lower middle income countries in some regions of the world. Despite the fact that mortality is relatively low in the adolescent years, it’s estimated that in 2012, 1.3 million 10 to 19-year-olds died, mostly from preventable or treatable causes. Mortality declines have been least in 10 to 14-year-olds and 15 to 19-year-olds in comparison with other age groups.

And therefore, the relative importance of mortality in adolescence has increased relative to other age groups. The net product of the changes in fertility and the changes in mortality result in the population pyramid or the age structure of the population changing. In this pair of graphs, we see the population pyramid for Switzerland, a high income country. And you can see that in 1950, there were far more young children under the age of 10 than there were adolescents in the 10 to 19 year age group. Whereas, by 2010, that pattern had been reversed and there are far more adolescents than there are children under 10.

If we now turn to South Africa and the current picture from 2010, you can see that there were roughly similar numbers of children in the first decade of life as they are in the second decade of life, the adolescents. And that’s projected to change such that by 2050, the number in the second decade of life will greatly outnumber the number of children in the under tens. So why is the demographic transition important? Well, it’s because it gives a one time potential economic dividend to countries because the population is replete with young adults who are in the most productive years of their life. But to realise this potential dividend, the young adults must be healthy, well-educated, and employed.

If not, they’ll fill our unemployment queues, hospitals, and streets with angry protesters. In parallel with the demographic transition, there’s an epidemiological transition that is occurring. And this is because infectious diseases have fallen in importance, especially in childhood. This is partly due to vaccination, insecticide treated nets, reducing malaria transmission, improved water supply, hygiene, and sanitation, the prevention of treatment for diarrhoea, pneumonia, malaria, TB, HIV, and other infectious diseases. This is illustrated here for diarrhoeal diseases where we’ve got a line for Africa at the top there, and for the world, in blue below it, which shows the deaths per 1,000 live births among children under five due to diarrhoea.

And you can see that between 2000 and 2011 there was a very substantial decline in diarrhoeal disease mortality in under fives. And the somewhat similar pattern was also seen for acute lower respiratory tract infections. On the other hand, injuries, mental health problems, and non communicable diseases such as diabetes and hypertension have increased in importance. And currently, the top five causes of death globally among adolescents are road injury, HIV, self-harm, lower respiratory infections, and interpersonal violence. So in summary, globally, one in six of the world’s population is an adolescent, with a higher proportion in low and middle income countries where the vast majority of adolescents live.

The importance of adolescent health has been increasing over the past decades for two key reasons. The first is the demographic transition which was due to falling fertility rates and huge improvements in child survival. And the second is due to the epidemiological transition which is being caused by declines in infectious diseases and the increasing importance of non communicable conditions.