The continuing confusion caused by comparing numbers of Covid-19 cases and crude rates internationally reflect a failure of public health sciences. Age and sex-specific rates have primacy.

Dear Editor

Title: The continuing confusion caused by comparing numbers of Covid-19 cases and crude rates internationally reflect a failure of public health sciences. Age and sex-specific rates have primacy.

Islam and Jdanov explain why age and sex adjustments are critical when comparing death rates concluding, “Crude comparisons might have had some value as “quick and dirty” reporting in the early days of a catastrophic global pandemic, but they must now be consigned to history.”[1] I disagree. Crude rates are inappropriate in comparisons when population structures vary and they created widespread confusion during the pandemic.

Foundational courses and textbooks emphasise examining age and sex-specific stratified rates and then, to produce summary statistics without confounding by these variables, adjusting rates using direct or indirect standardisation.[2 3] One just needs the age and sex structure of the populations for standardisation, easily available in most countries.

My plea for both age and sex specific and adjusted rates was published on 3/4/2020 (18/3/2020 for preceding rapid response; https://www.bmj.com/content/368/bmj.m1090/rr-5). I wrote “We are being misled about the potential dangers (or not) by using overall or crude death rates.” My colleagues and I showed the necessity of examining data by age group,[4] demonstrating the extremely low risk in children,[5 6] thereby influencing UK policy. We showed the higher mortality in males than females, especially in younger and older age groups.[7] Such variations would be lost by age and/or sex standardisation without first examining age and sex specific rates, which have primacy.[3] Epidemiologists are, after all and as the witticism says, broken down by age and sex. We also demonstrated major changes in variations in mortality after age and sex adjustment across regions of Italy [8] and across 25 European countries.[9] This body of work augments the points raised by Islam and Jdanov.[1]

It is a failure of public health sciences, especially epidemiology and biostatistics, that age/sex specific rates have had so little, and crude rates so large, an influence in the international dialogue in the pandemic.

Raj S Bhopal, Emeritus Professor of Public Health
Edinburgh Migration, Ethnicity and Health Research Group,
Usher Institute, Medical School, University of Edinburgh, Teviot Place, Edinburgh EH8 9AG

1. Islam N, Jdanov DA. Age and sex adjustments are critical when comparing death rates. BMJ 2023;381:p845. doi: 10.1136/bmj.p845
2. Bhopal R. Which book? A comparative review of 25 introductory epidemiology textbooks. Journal of Epidemiology and Community Health 1997;51(6):612-22.
3. Bhopal RS. Concepts of epidemiology : integrating the ideas, theories, principles and methods of epidemiology. 3rd edition ed. New York, NY: Oxford University Press 2016.
4. Olabi B, Bagaria J, Bhopal SS, et al. Population perspective comparing COVID-19 to all and common causes of death during the first wave of the pandemic in seven European countries. Public Health in Practice 2021;2:100077. doi: https://doi.org/10.1016/j.puhip.2021.100077
5. Bhopal SS, Bagaria J, Bhopal R. Risks to children during the covid-19 pandemic: some essential epidemiology. BMJ 2020;369:m2290. doi: 10.1136/bmj.m2290
6. Bhopal SS, Bagaria J, Olabi B, et al. Children and young people remain at low risk of COVID-19 mortality. The Lancet Child & Adolescent Health doi: 10.1016/S2352-4642(21)00066-3
7. Bhopal SS, Bhopal R. Sex differential in COVID-19 mortality varies markedly by age. Lancet 2020;396(10250):532-33. doi: 10.1016/s0140-6736(20)31748-7 [published Online First: 2020/08/18]
8. Gallo V, Chiodini P, Bruzzese D, et al. Age-and sex-adjustment and the COVID-19 pandemic – transformative example from Italy. International journal of epidemiology 2020;49(5):1730-32. doi: 10.1093/ije/dyaa139
9. Gallo V, Chiodini P, Bruzzese D, et al. Comparing the COVID-19 pandemic in space and over time in Europe, using numbers of deaths, crude rates and adjusted mortality trend ratios. Scientific reports 2021;11(1):16443. doi: 10.1038/s41598-021-95658-4