The H-index, a good indicator of the impact of infectious diseases

Who other than scientists would have thought of using the H-index as an indicator of the impact of an infectious disease on the health of a population? Nobody, obviously, since this index is generally used to measure the productivity and impact of research. This novel idea was recently recounted in an article in the journal PLoS One.

The H-index considers the number of publications by a scientist and the number of citations of those publications. In the case of a disease, the principle is the same, except that the publications in question are not those by a given author, but those concerning a given disease. Used in this way, the index demonstrates the scientific community's interest in a disease. However, it was not a foregone conclusion that that interest was representative of the impact of a pathogen on the health of a population.

To check the relevance of the index, Agnès Waret-Szkuta, a CIRAD epidemiologist, and her colleagues, compared it with the best* known synthetic indicator: DALY (Disability Adjusted Life Year), which reflects the sum total of years of good health lost as a result of either premature mortality or disability. It is quite complicated to calculate, and has so far only been defined for well-known, highly significant human diseases. "We revealed a clear correlation between the H-index and DALY for 27 infectious diseases. This correlation suggests that the H-index is a good indicator of the relative impact of diseases ," says Agnès Waret-Szkuta. However, the correlation curve revealed the difficulty of publication and the lack of resources allocated to research on certain tropical diseases: nematode infections prevalent in developing countries were all significant outliers in relation to the curve.

The correlation was only obtained for the relatively small number of diseases for which DALY has been calculated, and the H-index obviously involves a degree of bias, of which only some can be corrected by hand. However, the H-index has many other advantages. It is easy to obtain, by accessing scientific publication databases. A fortnight was all it took one person to define the H-index of 1414 human pathogens and zoonoses (infections transmitted from animals to people, and vice versa). Furthermore, it can be calculated automatically and recalculated regularly. The study team already think that a rapid increase in a given H-index may be an indicator of new emerging diseases.

The result published in PLoS One is the first stage in an EU project, ENHanCe (EraNet Health and Climate in Europe). The aim is to verify the commonly held belief that climate change will have a significant impact on infectious diseases. "We took a bottom-up approach, basing the project on two questions: which infectious diseases have the most significant impact on the health of a population? We can now answer this question using the H-index. And which diseases will be most affected by climate change? ", Agnès Waret-Szkuta explains. "By coupling the answers to these two questions, we will be better able to assess the impact of climate change on human health, and thus to support the drafting of health strategies ." The next instalment will be in March 2012, when ENHanCe will officially come to an end.

* According to the World Health Organization