Demographic and epidemiological transitions

SPEAKER 1: We’re now going to talk about demographic and epidemiological transitions and their importance for adolescent health. Adolescents represent 1/6 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 will be a substantial decline in the number of adolescents in East Asia and the Pacific over the next 35 years, whereas the numbers in sub-Saharan Africa will continue to increase substantially through to 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 such as 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 has been very little change over that same period. And 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. This is the 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.

Despite the fact that mortality is relatively low in the adolescent years, it’s estimated that in 2012, 1.3 million turned 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 was a roughly similar number 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-10s. 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 is an epidemiological transition that is occurring and that 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 malaria, diarrhoea, pneumonia, TB, HIV, and other infectious diseases.

This is illustrated here for diarrheal 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 2015, there was a very substantial decline in diarrheal disease mortality in under-fives. And a somewhat similar pattern was also seen for acute lower respiratory tract infections. On the other hand, injuries and mental health problems and non-clinical diseases such as diabetes and hypertension have increased in importance. And currently, the top five causes of death globally among older adolescents, that is in 15- to 19-year-olds, are road injury, self-harm, HIV/AIDS, interpersonal violence, and drowning.

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. 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.