This article explains why the current mortality rates of COVID-19 should not be compared between countries.

The novel coronavirus disease 2019 (COVID-19) continues to spread internationally. Worldwide, almost 700,000 cases and 33,000 deaths have been reported. The WHO estimates the global mortality rate to be 3.4% but this varies between countries and across ages.

However the current mortality rate of COVID-19 might be skewed which doesn’t allow for a direct comparison between countries. County-level COVID-19 mortality rates rely on the number of deaths scaled to the number of confirmed COVID-19 cases. The denominator should be the total number of patients diagnosed with the virus in a country, but this is hard to estimate. The WHO, the National Data, and other sources have provided data on the cumulative number of cases being placed as the denominator while estimating mortality rates, which may not be appropriate since testing strategies across countries vary. Some countries used the strategy of testing symptomatic individuals with probably a high viral shedding, while other countries also tested many asymptomatic people, leading to an increase in the number of cases.

Other estimates use the number of COVID-19 patients who require admission to hospitals as the denominator while estimating the current mortality rates of COVID-19. However, here again, caution is needed as the differences in rates may reflect the heterogeneity in: (i) the clinical management and care of COVID-19 patients which may vary across countries or (ii) the guideline of admission to hospital for patients with confirmed COVID-19.

The author of this piece suggests that the final denominator to calculate the COVID-19 mortality rate may not be available or may be unknown for now. Therefore, comparison between countries should require other metrics such as: (i) the comparability between healthcare systems; (ii) the population size; (iii) the number of days since the date of the first diagnosed COVID-19 case; (iv) the heterogeneity in testing strategies across countries, for example testing symptomatic individuals in France with mostly a high viral shedding vs. testing asymptomatic individuals in South Korea or Germany; (v) the guidelines of admission to hospital for patients with confirmed COVID-19; and (vi) the variability in assessing the number of deaths per day, for example the number of deaths mentioned in France corresponds to the number of deaths in hospital.