Copyrighted Material
Prologue A Science Named Epidemiology
are consumer of health services, practicing clinician, health professional in training, health journalist, epidemiologist, or epidemiology student, this book explains how and why epidemiology has evolved in 350 years from being a science of epidemics to being indispensable for the discovery of effective ways of preventing health harms, prolonging life, and treating diseases. To solve enigmas of health and diseases, epidemiology has become a science that compares groups of people. Group comparisons are a tool to study the health effects of many aspects of human life, including lifestyle factors such as tobacco smoking, lack of exercise, excess caloric intake, oral contraceptive use, alcohol; social conditions such as education, income, occupation, and neighborhood; and environmental exposure such as air pollution. Comparing groups is also at the core of the scientific methods needed to assess the efficacy of medical procedures for prevention and treatment. Here is an example of group comparison: take a group of people who exercise and another who don’t. For the following ten years, count all the new cases of heart attacks occurring in each of the groups. At the end of the ten years, compare the frequency of heart attacks in each of the two groups. If exercise protects, you would expect heart attacks to be less frequent among the active than among the inactive group. Thus, in its simplest form, a group comparison can be used to determine whether a disease occurs more often in a group that is exposed to some factor than in a group that is not exposed to that factor. W h e t h e r yo u
Copyrighted Material
x i v P R o lo g u e
Most of the time, performing a group comparison is the only option to identify causes of diseases or whether treatments work. Think of all you know about the health effects of tobacco, diet, physical activity, occupational exposures, screening tests, drugs, medical care, social and economic living conditions, and other aspects of your life that may impact your health. Think of questions such as: What is a healthy diet? How many times per week should we exercise? Is this drug effective? Should we put babies to sleep on their belly or on their back? And so on for the health effects of sunscreen, alcohol, tobacco, drugs, contraception, and safe sex. If some scientific knowledge exists about the health consequences of these issues, it most likely is a find based on group comparisons. I use the term knowledge as opposed to beliefs. Don’t read it as a synonym of truth, but rather of evidence. I prefer to speak of knowledge than of evidence. Group comparisons produce data. These data are tangible; they can be evaluated, criticized, and then confirmed, or not. Whether the results of a group comparison are confirmed or not, whether they were true or untrue, we have learned something, we are more knowledgeable and can progress. We have the option of getting more data, better data, different data to improve our knowledge. In contrast, beliefs are products of our imagination. They are mere opinions with which we can agree or disagree, but whose origin we cannot evaluate. When I say cigarettes don’t cause cancer because my neighbor who is now ninety-five has smoked since he was fifteen and does not have any cancer, I express my belief based on my interpretation of a selected observation. You may be of a different opinion because a good friend of yours, a smoker, died at age fifty of lung cancer. We may argue forever, no one being able to tell who is right or who is wrong until we can rely on knowledge. This example may seem mundane, but many, many circulating health statements on important issues, such as vaccines, are of that sort. I am not saying that we should discard opinions and beliefs. As the nineteenth-century German professor of hygiene Max von Pettenkofer used to say, “If we had to live only on what has been ascertained scientifically, all of us, as many as we are, would have perished long ago.”1 We need health beliefs. They are part of us and cannot be dissociated from us. My point is that it is important to distinguish knowledge from belief.
Copyrighted Material
P R o lo g u e x v
Group comparisons can help us do that with regard to health issues. Most of the health claims we find on the Internet or among our acquaintances are beliefs. Considering these beliefs on a par with knowledge can be a source of unwarranted anxiety or of ill decisions. My claim in this book is very simple and unpretentious: checking whether a health statement is based on a group comparison or not is an effective way of separating health knowledge from health belief. Epidemiology is already important in our everyday life. Where does the knowledge supporting the warnings on packs of cigarettes about the dangers of tobacco come from? Epidemiology. Why do you trust that the screening test recommended by your doctor can contribute to your living longer? Epidemiology. Which science provides the methods to determine whether surgery is effective for lower back pain? Epidemiology. In 2009, David Leonhardt asked Barack Obama “how going to the doctor will be different in the future; how they will experience medical care differently on the other side of health-care reform.” In his answer, the president emphasized “the importance of using comparative-effectiveness studies as a way of reining in costs” and the possibility for doctors to say to patients: “You know what, we’ve looked at some objective studies out here, people who know about this stuff, concluding that the blue pill, which costs half as much as the red pill, is just as effective, and you might want to go ahead and get the blue one.”2 What are the “comparative-effectiveness studies” that President Obama mentioned in relation to the health-care reform? Epidemiology. Thus, epidemiology is the science underlying most of the practical knowledge about whether measures to prevent and treat illness work. There is, of course, a wealth of other health knowledge that is not based on epidemiology, related to the anatomy, physiology, and pathology of the human body; to the cellular effects of drugs or microorganisms; to the molecular structure of the toxicological and infectious causes of diseases; and so on. The full list of all the domains in which health knowledge progresses without resorting to group comparisons would be very long. But rarely do they provide knowledge that is immediately relevant for prevention, treatment, or screening.
Copyrighted Material
x v i P R o lo g u e
Epidemiology in the Media
The results of group comparisons can be found daily in the health sections of newspapers and magazines. Here are some titles gleaned from English-speaking media: “Walking Six Miles Each Week Could Reduce Chance of Getting Alzheimer’s” (Daily Mail, United Kingdom, November 29, 2010)3 “Cell Phones and Cancer: A Study’s Muddled Findings” (Time, May 17, 2010)4 “People Who Drink as Few as Two Soft Drinks a Week Face Nearly Twice the Risk of Developing Deadly Cancer, Study Finds” (CBSNews Healthwatch, February 9, 2010) “Diet: Eating Fish Found to Ward Off Eye Disease” (New York Times, March 22, 2011) “Scientists Identify Genes for ‘Extreme Longevity’ ” (AOL News, July 1, 2010) “Alzheimer’s Gene ‘Linked to Vitamin D’ ” (Telegraph, May 30, 2011) “Politicians Should Not Prescribe Pills” (Financial Time, July 14, 2010) “Progestin: Hormone Replacement Therapy Study Halted” (CNN Health, July 9, 2002) “Good Week for People Who Make Their Grievances Heard at Work” (The Australian, May 28, 2011) “Vitamins ‘Lower Risk of Autism’ ” (Sydney Morning Herald, May 27, 2011) “Research Shows Extra Calcium Unnecessary” (The West Australian, May 25, 2011) “Study Shows Caffeine Might Prevent Pregnancy” (Ottawa Citizen, May 25, 2011)5
All of these articles comment on the results of group comparisons performed using epidemiologic methods. Some news agencies have specific epidemiology sections on their websites, reporting the results of group comparisons.6 Even though epidemiology’s importance has grown in our everyday life, I suspect many people do not recognize it every time they come across its results. Several explanations come to mind for this relative lack of visibility.
Copyrighted Material
P R o lo g u e x v i i
Epidemio-logy
Most medical specialties are named according to their subject matter: cardiology deals with the heart, gastroenterology with the stomach and the gut, neurology with the nervous system, and so on. The same is true for most human and social sciences, such as sociology, psychology, economics. What is epidemiology about? When I introduce myself as an epidemiologist, people sometimes seek advice for a skin problem because they think I am an “epidermologist.” More often they will ask: “Oh, you must be knowledgeable about the HIV epidemic?” To which I reply, “Not particularly, sorry.” The fact is that equating epidemiology with the science of epidemics is etymologically correct. The term epidemiology literarily evokes a medical specialty that deals with contagious epidemics. Indeed, in the nineteenth century epidemiology was the science of epidemics. The recurrent epidemics of cholera, a scourge that periodically ravaged the world, stimulated the creation of a professional association of epidemiologists in 1850 called the London Epidemiologic Society.7 Today’s epidemiology, however, does not exclusively study infectious diseases. Its scope has expanded to the identification of all determinants of human health, whether they occur as epidemics, with rapidly changing frequencies over short periods of time, or not. Epidemiology is apt to study the health effects of many characteristics of human life. It also provides the science needed to assess whether medical procedures, treatments, and screening tests are salutary. The examples used in this book and the headlines mentioned earlier give an idea of the breadth of topics to which epidemiology can be applied in order to assist in the acquisition of health knowledge.
Methods Versus Matter
Why did epidemiology keep its nineteenth-century name if it no longer applies? Maybe because it is challenging to give a name to a mode of research characterized by the use of group comparisons. Epidemiology is not associated with a particular domain of medicine or public health.
Copyrighted Material
x v i i i P R o lo g u e
All domains of medicine and public health rely on epidemiology. In the “war” against cancer, the “battle” against polio, the “fight” against obesity and alcoholism, or the “struggle” against health inequalities, epidemiology provides the strategists, not the domain experts. Experts are, respectively, cancerologists, vaccinologists, nutritionists, alcohologists, and social scientists. Some experts may also be epidemiologists, and epidemiologists may also be domain experts, but the strictly epidemiologic component of the endeavor is to identify the populations to be compared and the modes of collecting, organizing, and interpreting the data. The crucial role of epidemiology can elude the public because its methodologic contribution remains in the background when the results of a study are disseminated. The discovery of an anticholesterol drug that decreases the risk of heart attacks appears as a success of cardiology or lipidology rather than of epidemiology even though the design and conduct of the study relied on epidemiology.
Science and Know -how
Because epidemiology has an ambiguous name and an activity often out of public view, its contribution to health knowledge is not always obvious. On top of these obstacles we add its theoretical complexity. Comparing groups requires a scientific training and a know-how that pertain to epidemiologists. Are experts really needed to identify comparable groups? The answer is yes. Epidemiologists are experts at identifying and recruiting groups of people that can reasonably be compared. This is not a trivial activity. Some group comparisons have simpler designs than others. For example, there are experimental designs in which a potentially beneficial treatment is attributed to some patients but not to others using a chance procedure equivalent to tossing a coin. In such randomized controlled trials (a term explained in chapter 8), it is easy to perceive that the groups will be comparable. A great deal of the difficulty is technical and lies in the performance of the experiment. These trials are ideal for evaluating whether treatments have beneficial health effects or not. At the same time, it is not possible to toss a coin to allocate a potentially deleterious exposure or behavior. Would you prescribe either a pack of cigarettes containing true tobacco
Copyrighted Material
P R o lo g u e x i x
or cigarettes free of tar and nicotine to (up to then) nonsmokers in order to determine if the health of those who smoke true tobacco declines faster than of those who smoke fake tobacco? Of course not, and therefore the effects of tobacco products on health need to be studied with people who have already chosen to smoke or not. Both groups differ substantially beyond their smoking habits. They live different lives and have different motivations for participating in epidemiologic studies. How can we reach out to them? How can we ensure that the findings among people who agree to participate in the study are relevant for the population at large? These situations cry for creative study designs; otherwise, the results will be fraught with error and invalid. Let me use here an example from my own experience. My colleagues and I conducted one of the first group comparisons of whether women who smoke cigarettes or who breathe air polluted by cigarette smokers were at increased breast cancer risk.8 Jane Brody of the New York Times asked, “Could cigarette smoking account for the mysteriously rising incidence of breast cancer among American women?” She went on to describe the design of the group comparison: “The study, conducted among 244 women with breast cancer and 1,032 women free of the disease, revealed that the more a woman smoked, the greater were her chances of having breast cancer. Thus, for women who smoked less than half a pack a day, the breast cancer risk was doubled; for those who smoked 10 to 19 cigarettes a day, the risk was 2.7 times greater, and for those who smoked a pack or more a day, the risk was 4.6 times greater.”9 In that study, we carefully tried to recruit all new cases of breast cancer in the population over a two-year period. We simultaneously drew a random sample of the general population to serve as comparison group. We interviewed the participants about their lifelong exposure to active and passive smoking. The fact that smoking was more common in women diagnosed with breast cancer than among comparable women who weren’t did not necessarily imply that smoking caused breast cancer. Because breast cancer is more common in women who never had children, we needed to make sure that this was not the reason for the observed difference. If nonsmokers have more children, then that might be the explanation behind their lower risk of breast cancer. The same reasoning applied for all the known and postulated causes of breast cancer. There are alternative
Copyrighted Material
x x P R o lo g u e
explanations and methodological errors to consider before presenting an observed relation as potentially causal. The conduct of group comparisons requires a theoretical background taught in schools of public health10 and a technical know-how acquired through practice. This book is not about epidemiologists’ theoretical background or know-how.
This is Not a Textbook
The book you are reading is not a “popular” textbook of epidemiology because no technical expertise in these methods is required to use epidemiology in everyday life. When epidemiologic results become accessible to the public in newspapers, in popular media, or on the Internet, they usually have already been deemed valuable by the scientific community. Their authors have, in principle, already convinced the scientific journal editors and experts that their findings were rigorous enough to be published and shared with others. At that stage, when results become news for the public, the technical aspects of the group comparison are not crucial anymore. Readers of public news media need to be able to understand how the groups were compared, who was compared, and how much health benefit or health harm was observed. This book addresses the public’s need. Even knowledge of math is unnecessary. Of course, epidemiologists use statistical techniques to analyze the data they collect. These methods can be quite sophisticated, but the users of a statistical analysis do not need to understand the mechanics of that analysis to be able to interpret the study results. The statistical component of epidemiologic studies is rarely if ever mentioned in the news coverage of epidemiologic findings. It is extremely unusual for a newspaper to mention whether “the number of participants was large enough to detect the effect of a treatment or of a preventive factor if it existed” or to provide “the probability that similar or more extreme findings would have been observed in another study if there were no association.” Epidemiologic and statistical expertise is needed before the results are disseminated. It is not crucial after that point. I hope to convince you of that.
Copyrighted Material
P R o lo g u e  x x i
However, the concepts explained in my book, such as population thinking and group comparisons, are noble. I did not try to vulgarize them or simplify them in order to make them accessible, as I would have had to do for statistical or technical issues. What you can learn in this book about epidemiology you will not have to learn again or in a different way. But if your interest in epidemiology has been piqued by this book, I recommend that you consult a more traditional text that presents the basic concepts and methods. Overall, epidemiology’s great contribution to health care and public health lies in a fairly simple discovery: group comparisons are indispensable to acquire health knowledge about prevention, treatment, and screening efficacy. Because this truth does not make intuitive sense, it took several centuries and many enigmas of health and disease to become commonly accepted. It is the historical sequence of some of the scientific mysteries epidemiology unraveled that I will replay in the coming chapters.
Timeline
For those who wonder how to approach the chapters of this book, here is some information that may be useful. First, regarding the order in which the chapters can be read. Each chapter stands alone, but there is a timeline across chapters 2 to 13. Chapter 2 goes until the end of the sixteenth century; chapter 3 covers the seventeenth and eighteenth centuries; chapters 4 and 5 cover the nineteenth century; chapter 6 covers the first half of the twentieth century; chapter 7 covers the second half of the twentieth century; and chapters 8 to 13 are about the end of the twentieth and beginning of the twenty-first centuries. In order to avoid technical digressions in the main text, I have added four appendixes, written in the same style as the rest of the book, for readers who want to follow the technical discussion in more detail. Finally, the notes link the text to the bibliography and allow you to find the sources or explore a topic further. In the notes, articles and books are cited by the author’s name followed by the year of publication (e.g., Snow 1855) because the full reference can be found in the bibliography.
Copyrighted Material
x x i i P R o lo g u e
Conflict of Interes t
People who read the French edition of this book have asked me where the financial resources to write and publish it came from. Did some industry support me? Could some of my statements be tainted by the need to protect my sponsors’ interests? I found these questions absolutely legitimate. After all, I used to ask every person who published in Preventive Medicine, the journal I was chief editor of, to sign a conflict-of-interest form disclosing any link to corporate interests, which might alter the researcher’s independence. Why would I not be similarly obligated to my readers? This disclosure is quite simple. I did not receive support from corporate sources. I used the material and documents from the lectures I give in various academic settings and wrote the book mostly during my spare time. A 2012 three-year grant from the US National Library of Medicine has allowed me to dedicate much more time to study the history of epidemiology. Even though the National Library of Medicine project is not directly related to this English adaptation of Santé, it has definitely increased my expertise in the topics covered in this book. My publishers, Odile Jacob in France and Columbia University Press in the United States, have been responsible for the cost of publication of this book.