A commentary on Khoury & Galea Will Precision Medicine Improve Population Health

The last year has seen much by way of commentary, both positive and negative, on precision medicine and its opportunities for improving health. A new Viewpoint article in JAMA from a leading proponent of public health genomics and an academic public health physician attempts to define its pros and cons.

I have some slight concerns about the approach taken in this paper and hope that outlining these (together with some comments on the paper itself) may add a new dimension to this important debate.

Distinguishing individuals from populations

First, I believe that it is not useful and possibly even divisive to polarise the debate in this manner, to pit population health against the health of individuals. The paper states explicitly that 'the health of individuals or small groups of individuals are not addressed here because it involves a different question'.  A population is no more than a group of individuals. Populations can be defined however we wish, by reference to age, sex, country of origin, or whatever parameter we chose to invoke. The concept of population is thus a social construction. By contrast, individuals are an undoubted reality. Biology works through individuals, and to the extent that the wider population determinants of health (including social and economic determinants) influence the outcome of a population’s health, they do so through biological mechanisms expressed through individuals in that population [1]. There is a second difficulty: it is logically incoherent to establish a rational distinction between a population and a small group. What criteria might be used to say of a group of individuals, this is a small group but it is not a population?  

There is, by contrast, a real and important distinction to be made that is absent from the paper: between the interventions that are used to improve population health and the interventions used to improve the health of individuals. I suggest that there are two categories of interventions. There are interventions that are directed at the external environment without reference to any particular individual (such as legislation or advertising or specific action on poverty, housing or the water supply), where the intention is to improve that environment (whether physical, social, biological or economic) for all individuals that are exposed to it. There are other interventions that are directed at individuals and intended to achieve its effect through the agency of those specific individuals. Precision medicine is this second type of intervention, since it is targeted at individuals and can only achieve its effect if those individuals comply with what is offered to them. This distinction that I draw and commend is therefore not the distinction that the authors make between groups of individuals and the population, but rather that between those directed at the external world and those directed at the individual [2].

Predicting risk amid complexity

Second, I fail to understand why the complexity of disease pathogenesis should be a factor that would any more impede the efficacy of precision medicine than any other intervention in improving population health. It is of course entirely true that population-based prevention strategies, by shifting the normal distribution of disease risk to the left (for example, lowering population blood pressure by reducing dietary salt intake) will be more effective at a population level than high-risk prevention strategies (of which precision medicine is an example). However, this neither supports the view that this is a result of complexity, nor does it negate the utility of using ’complementary individual and public health approaches to health care and disease prevention‘ as the authors themselves suggest.  

The statement that ’genetic associations have… small effect sizes‘ is immaterial, since nobody with any understanding of the field would claim that individual genetic variants are of any use in risk prediction of common complex disorders. The more accurate claim is that it is the use of risk prediction algorithms incorporating multiple biomarkers and environmental factors that will provide the greatest utility in identifying individuals at the greatest risk of disease [3]. Incorporating an understanding of gene-environment association will also probably pay dividends in risk evaluation, since a population showing that genetic variant X has an effect size of 1.18 may in fact hide the fact that its effect size on 95% of the population not exposed to environmental factor may only be 1.03, but for the 5% so exposed, the effect size may be 4.0.

A united vision of health

Third, the assertion that current data suggest that ’individuals do not change their behaviour much‘ even when they become aware of being in a high-risk group is based on a single meta-analysis based on flawed data, and predicated on the assumption that it is only genetic variants that are of relevance. In my view, the data do not justify the conclusion drawn by the authors. I will say little more about this since I have commented in an earlier blog [4].

There are other contentions in the paper that speak to the assertion that precision medicine is unlikely to improve population health. These include the undesirability of the shift in funding from public health to -omic led projects and the hype of the promise of precision medicine. Whatever the truth or otherwise of these assertions, they do not in my view provide any argument either for or against the role of precision medicine. By contrast, there is strong evidence for its efficacy in improving the health of people with certain high penetrance genetic disorders and in infectious disease control.

Yet, at its core, my argument concerns the sterility of pitting the public health approach against precision medicine. Both are valid and useful. The authors are clearly aware of this in stating that ’health professionals need greater emphasis on joining biological with social and environmental determinants of health‘; and on the use of ’emerging information from multiple levels…to develop a better understanding of determinants of health‘.  If ’there are clear tensions at the intersection of precision medicine and public health‘, I suggest that they are entirely of our own making. Population health and individual health are different ways of looking at human health, while prevention and treatment are a continuum that defies easy or rational separation.  

I suggest that we no longer make these distinctions and regard all types of interventions that seek to improve health, whether through prevention or treatment and whether directed at the external environment or at individual directly, as valid and desirable. The genomic and precision medicine revolution can unite what has set us apart for many decades.  

References

1. Ernst Mayr (1975) Typological versus Population Thinking.  From Evolution and the Diversity of Life, Harvard University Press, 1975, 26–29.

2. Hilary Burton & Ron Zimmern (2015).  Personalised Medicine and Public Health: An Urgent Agenda (blog)

3. Khoury M (2014).  Geography, Genetics and Leading Causes of Death (blog)

4. Hilary Burton, Leila Luheshi, Ron Zimmern and Alison Hall (2016) Complex diseases need complex solutions – including genomics (blog)