Epidemiological Transition

Epidemiological Transition Stages

In 1971, ET theorist AR Omran, in a bid to build upon and improve demographic transition theory, proposed three epidemiological shifts over the last 12000 years that resulted in "ages."¹ Two more ages (stages) have been added since then.

The first age was sparked by the Neolithic Revolution when people became farmers, living sedentary existences near each other and their animals. Diets worsened in many ways as they lost access to the range of wild foods hunter-gatherers consumed.

Sedentary farmers and urban dwellers became highly susceptible to zoonotic transmission of disease from domesticated animals as well as commensal rodents such as rats and mice, highly effective disease spreaders.

First Age: Pandemics and Famines

Until 1492, this age of "pestilence and famine"¹ was experienced by farmers and urbanites in the Old World. Hunters and gatherers who remained uncontacted were not directly affected. After 1492, pandemics and famines were the norm across the globe among all farming and urban people.

Life expectancies during these 12 millennia ranged from 20 to 40 years, due to diseases but also wars and famines, which people dependent on agriculture experienced when crops failed.

Long-term, populations fluctuated in cycles. In times of abundance and peace, populations grew, but then they crashed when a new pandemic or famine swept across the land.

Fig. 1 - 'The Triumph of Death' (1562) by Pieter Bruegel the Elder depicts a personified version of the Black Death, caused by a virus transmitted by rat fleas

Women, infant, and child mortality were extremely high during this entire period, with up to one out of two children dying before the age of two.

Eventually, modern medicine, healthcare, and sanitation ushered in the next age.

Second Age

The dawning of the modern era and the Industrial Revolution in the 1600s and 1700s AD, in Europe and North America and later elsewhere, saw several developments that helped to prolong life expectancy and reduce birth rates as well as death rates. This was Ohran's "Age of Receding Pandemics."¹

Fig. 2 - Snow's map of cholera cases in London

Discovery after discovery, law after law, and treatment after treatment helped raise the average life expectancy to 55 years at birth. Childbirth became safer for mothers, children became healthier, particularly with vaccines, and populations skyrocketed as a result. This is equivalent to the second phase of the Demographic Transition when populations grow exponentially.

Third Age

Once penicillin began to be used to treat bacterial infections in the 1940s, it can truly be said that a transition had occurred and a third age had dawned. Ohran called this the "Age of Degenerative and Manmade Disease."

This age is characterized by non-communicable diseases (NCDs) such as cancers and heart diseases. These are sometimes called "diseases of affluence" as they are the prime killers in countries with advanced economies where people have access to the full range of vaccines and treatments for transmissible diseases, so these have largely been conquered, and public health and sanitation are at high levels. Life expectancy ranges into the 70s, and infant as well as maternal mortality drop to negligible levels.

Not all societies today have fully entered this age. Many are stuck in the prior age because they still experience relatively high infant and maternal mortality, low life expectancies, and many preventable transmissible diseases, such as cholera, malaria, dengue, and so forth.

Fourth and Fifth Stages

Omran added the "Age of declining cerebrovascular mortality, aging, lifestyle modifications, and resurgent diseases" in 1983.¹ Cardiovascular disease death tolls have been reduced thanks to healthier lifestyles (e.g., less smoking, better diets, and less air pollution) and better healthcare. Dominant killers are now the diseases of old age, which in earlier times had had a negligible on the overall population because few people lived that long. Now, countries in this stage have average life expectancies that top 80.

A fifth age (or stage) that some identify has seen newly-emerging infections such as HIV/AIDS, diseases associated with obesity, and the reemergence of diseases thought to have been vanquished, such as tuberculosis and malaria. The causes for these are varied, and we should now add coronaviruses such as COVID-19 to the list. The US seems to be at this stage.

Epidemiological Transition vs Demographic Transition

The epidemiological transition shifts the primary reasons for population growth from socioeconomic to epidemiological. In other words, the type and intensity of diseases are seen as the main drivers of population change rather than factors and forces such as wealth or poverty.

The stages of the demographic transition remain the same, but ET theory allows us to generate different models for different countries. Ohran's original models were:

Western Model of Epidemiological Transition

The transition from high to low death rates and high to low birth rates happened simultaneously and over 150 years during the Industrial Revolution in western Europe and North America. As a result, the natural increase of populations leveled out by the mid-20th century. Populations in what became known as the Developed World enjoyed all the benefits of modern medicine, healthcare, and sanitation as they emerged, though setbacks such as major wars and pandemics like the Spanish Flu of the late 1910s also occurred.

Accelerated Model of Epidemiological Transition

Japan is the prime example of a country that moved quickly through the entire demographic transition, skyrocketing from "medieval" to modern conditions in around 50 years. Other countries in East Asia, such as South Korea and Taiwan, also went through rapid modernization with an accelerated effect on mortality in the 20th century.

Delayed Model of Epidemiological Transition

Many countries have not gone completely through the demographic transition as was experienced in the developed countries for a variety of reasons. These include a continuing emphasis on large family size, notably in predominantly Roman Catholic and Muslim countries, for example, where birth control is either frowned upon or forbidden.

Epidemiological Transition Model Strengths and Weaknesses

All models attempt to describe past and current conditions to allow accurate predictions of future scenarios. People who make models choose which variables to include and which to leave out. The models generated by ET theory have been praised as well as criticized for what they get right as well as what they get wrong.

The major strength of ET theory is its focus on disease, health, and sanitation as leading variables in determining who lives and who dies and at what age.

Probably the principal weakness of ET theory is that it is too generalized. It is now understood that demographic models need to look at factors of race, sex, ethnicity, socio-economic status, and so forth to interpret patterns of mortality and disease.

Fig. 3 - HIV/AIDS adult prevalence map shows the highest rates (over 15%) in southern and central Africa, associated with cultural as well as socioeconomic factors specific to those regions

Epidemiological Transition Example

The US is an example of a country that has gone through all five stages of the epidemiological transition.

People in the US began to emerge from the first stage by the early 1800s as the country became less agricultural and more urban and industrial. But not everyone: areas such as the Deep South, and populations such as Native Americans and African Americans, lagged considerably behind in the arenas of disease control, healthcare, and sanitation.

Still, by the second half of the 20th century, the US population overall had transitioned from preventable, transmissible diseases to non-communicable diseases such as cancers and heart disease as leading causes of death. Infant mortality and maternal mortality were among the lowest in the world.

New Century, New Ills

With 50 million or more people in the US still living in poverty, and steady rises in problems such as drugs, homelessness, and depression, combined with the unhealthy diets promoted by the food industry (processed food and fast food), diseases such as those associated with obesity began to skyrocket around the turn of the 20th century.

The US had left the supposed last stage of the demographic transition and the 3rd stage of the epidemiological transition and entered uncharted ground.

New pandemics such as HIV/AIDS and COVID-19 swept the country. Depression leading to suicide and linked to abuse of narcotics as well as prescribed medications and many other factors have led to many deaths. Diets have remained high in fat, sodium, sugar, and other killers, leading, for example, to rapid increases in Type II (adult-onset) diabetes. With a steadily aging population, neurological diseases such as Alzheimer's and Parkinson's became greater factors.

The US's rate of natural increase declined due to this. The sky-high cost of healthcare didn't help. Despite access to public health options, treatments for many diseases became limited to those with good health insurance plans, which meant those higher on the socioeconomic ladder. The result has been a drop in life expectancy, from around 79 to 76.