Time for Cohort Studies in Africa

Africa, a continent of nearly one billion people with the world’s highest mortality rate at every age group and from nearly every cause, has no in-depth, large-scale longitudinal studies of its people’s health. No studies similar to Harvard University’s Nurses’ Health Study, which has studied lifestyle factors of 121,700 female nurses for 33 years, down to what they drink and eat, how much they exercise or smoke, and detailing their family and reproductive histories exist. Harvard, with its unmatched experience in this and other large cohort studies, is uniquely placed to share this knowledge with African colleagues, and to jointly learn from them.

 This lack represents a critical information vacuum in Africa, a continent being hit with a double dose of disease. Infections including tuberculosis, malaria, and HIV/AIDS have been seen as Africa’s major health burden. But now, in addition to these, there is a rising epidemic of chronic, non-communicable diseases such as heart disease, mental illnesses, trauma, cancer, diabetes, and obesity. Chronic diseases are projected to cause more deaths in the region than infectious diseases by 2030.

The shift in disease burden has a number of root causes. Victories against infectious disease mostly affecting children lead to more people living to adulthood, and thus an increase in chronic diseases. Economic development leads to lifestyle changes that increase the risk for chronic diseases. For example, shifts from agrarian to urban living results in less physical activity, more processed food, and exposure to more air pollution. Most African governments lack information and resources to respond to this growing crisis. 

A cohort study enrolls a defined population and follows them over time.  Besides the aforementioned lifestyle choices, participants may also be asked about environmental and occupational exposures, mental health, social relationships, living conditions, and economic circumstances. Biological samples such as blood, saliva, urine, and nail clippings can be collected to measure toxins, hormone levels, or genes. These observations over time allow for an in depth understanding of the reasons for health and risks for disease. A great advantage of cohort studies is that they enable scientists to study multiple diseases (for example heart diseases, cancer, stroke) and multiple risk factors (diet, exercise, air quality) over an individual’s lifetime.

Cohort studies have proved crucial in understanding non-communicable diseases in the US. A primary example of the public health impact of cohorts is knowledge about the heart-clogging effects of trans fat, something common in processed foods. Information gleaned in part from Harvard cohorts led to mandatory trans fat food labeling in 2006, and its subsequent ban from restaurants in New York City, Philadelphia, Boston, Cambridge and California. 

The regional disparity in cohort research is enormous. Taking into account population, cohort studies of Africans have produced a tiny fraction of medical research articles compared to wealthier areas. For every one article on high blood pressure published on Africans, 9 are published on Europeans, and 50 are published on people from the US. Disparity in the actual numbers of adults enrolled in cohorts is even worse. Taking into account population, we estimate that for every African enrolled in a cohort study, there are 190 Europeans and 1,000 Americans enrolled in cohorts.
  
We do not doubt that smoking, obesity, high salt intake, sedentary lifestyle, and pollution will have similar adverse health effects in Africa as they do in other places. Why then do we need cohort studies in Africa? We suggest at least five reasons. First, there may be unpredictable interactions between the simultaneous infectious and non-communicable disease epidemics unique to Africa. Second, it is important to determine population specific disease burdens in a region with few birth and death records and other health statistics. Third, humans evolved in East Africa, making African genetic diversity greater than elsewhere in the world. Studying the interaction between environmental factors and genes elucidates disease mechanisms. Fourth, the unparalleled geographic, social, and cultural diversity may reveal risk factors as yet unknown. The final reason is to generate locally relevant results to stimulate political will for health promoting policy. 
 
The Harvard School of Public Health and African scientists have begun an ambitious project to start cohorts of 100,000 people in each of 4 countries covering west, east, and southern Africa. Doing so would reduce the US-Africa disparity in cohort enrollees from 1,000-to-1 down to 20-to-1.  Cohort studies require substantial investment in personnel and infrastructure, but give immeasurable returns. These studies can run for decades, so in addition to research objectives, they build local capacity for future research by training students and scientists, making the effort locally sustainable. Thus far, international organizations and funders have overlooked this urgent need.  

We believe that large cohort studies are a proven design that will serve African public health, and could also yield information relevant to the chronic disease epidemic in the US. The bold African Cohort Initiative aspires to fill this knowledge gap, and seeks equally visionary funders to bring studies that have enormously benefited wealthy countries to Africa. The time for action is now, before the problem escalates, so that results can guide successful prevention programs.


Michelle D. Holmes, MD, DrPH is an associate professor of medicine at Harvard Medical School and an associate professor of epidemiology at the Harvard School of Public Health. Shona Dalal, PhD is a research associate at the Harvard School of Public Health.