Coronovirus Outbreak: COVID-19 mortality rates and age demographics; what the data tells us
Editor's Note: Data on proportion of population above the age of 65 is from World Bank for the year 2018 (the latest available on the website). Data on confirmed cases and deaths from www.ourworldindata.org (downloaded on 27 March). Plots were generated by the author, based on data downloaded on 27 March, 2020. All statistical analysis (including coding) has been done by the author.
A lot has been said about mortality rates in the context of the novel coronavirus outbreak. We know that most of the cases of COVID-19 that resulted in fatalities belong to the older age group, roughly above the age of 65 years. From that, it is usually assumed that countries with a higher proportion of elderly people are bound to see a higher mortality rate. For the sake of clarity, throughout the article, mortality rate refers to the proportion of the total number of deaths due to COVID-19 out of the total number of confirmed cases. This article examines that claim based on publicly available data.
There are two primary issues with the assumption. First, the mortality rate for different countries is still not clear; generally speaking, it has been rising steadily. Second, as per the available data, there is no statistically significant relation either between the mortality rate and proportion of the population above the age of 65, or between mortality rate and the median age of the population across different countries.
As per the trends so far, the mortality rate for most of the countries has changed significantly over time. For most of the countries, it has been steadily increasing. The United States is a major exception to this trend. The decrease in the mortality rate for the United States may, however, be attributed to poor testing initially that resulted in very few confirmed cases (even though the actual number of cases were probably much higher), which increased the mortality rate in the initial period.
We, therefore, still do not know the mortality rate of COVID-19 for different countries with complete certainty. A decrease in the number of new cases and deaths from older cases could be a reason for an increase in the mortality rate. However, generally speaking, the trend is consistent with many countries that are witnessing an increasing number of newer cases every day – like Italy, Spain, Netherlands, Belgium, and Japan, to name a few.
Even if, for the sake of a rough estimate, we take the current mortality rates as final, there is another major problem. It would be erroneous to directly compare the mortality rates (as of now) and demographics of countries, as different countries are at different stages of the COVID-19 outbreak. For example, it is disingenuous to compare the current mortality rate of Italy with that of India, as both are currently at different stages of the outbreak.
In order to control for that, the mortality rates have to be compared at roughly the same number of cases (per million, to control for population size). In other words, it means comparing the mortality rates of different countries at a certain fixed number of cases. I have considered different levels of the number of cases per million — roughly (0.57), (1.4), (2), (3.3), (5.2), (8.7), (13.2,) (21.7), (40.6), (78.9), (192.5), (221.4), (332.7), and (500) confirmed cases per million — to understand a general trend and make sure that the results are not unique to a specific level.
The next step was to investigate the relationship between the mortality rates of different countries (at each of the above-mentioned number of confirmed cases per million) and the proportion of population above the age of 65. The general expectation was that the mortality rates would be higher for countries with higher proportions of the population above the age of 65 across the different levels of confirmed cases.
The statistical analysis, however, did not show any statistically significant relation between the two systematically,based on 95 percent confidence intervals. None of the cases had a statistically significant positive correlation. Instead, the association was negative and statistically significant (i.e. lesser mortality rate in countries with a higher proportion of population age of 65) in one of the cases. A lot should not be read into it, as it was just one case and not a general trend. A similar analysis looking into the relationship between mortality rates and the median age of the population showed similar results.
How do we interpret these results? It is a fact that most of the COVID-19 cases that resulted in fatalities were those of older people. Yet, the variation in mortality rate is not simply explained by the age distribution of the population. It means that there are other significant variations at the country level that are causing variations in the mortality rate.
One obvious limitation of this exercise is that the data is for the number of confirmed cases as reported by the authorities of the respective countries. There may, however, be under-reporting or significant difference in the number of tests being conducted in different countries. As a matter of fact, there is a huge variation in the number of tests per million of the population in different countries. Nevertheless, this is the data available as of now, and it is likely a reflection of the actual trends – prima facie at least.
The variations at the country level are significant enough to make age distribution of the population insignificant for directly predicting the mortality date. Factors that actually affect mortality rate could include healthcare facilities, the extent of social distancing observed, immunity of the population, and so on. It is now for healthcare experts to probe it further.
The author is available at firstname.lastname@example.org for any questions/clarifications on data, method, or analysis.
Updated Date: Apr 03, 2020 11:42:19 IST
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