COVID-19: how to make between-country comparisons.

BACKGROUND: Different countries have adopted different containment and testing strategies for SARS-CoV-2. The difference in testing makes it difficult to compare the effect of different containment strategies. We propose methods to allow a direct comparison and we present the results of this comparison. DESIGN: Publicly available data on numbers of reported COVID-19 related deaths between January 1st and April 17th 2020 were compared between countries. RESULTS: The numbers of cases or deaths per 100,000 inhabitants give severely biased comparisons between countries. Only the number of deaths expressed as a percentage of the number of deaths on day 25 after the first reported COVID-19 related death allows a direct comparison between countries. From this comparison we observed clear differences between countries, associated with the timing of the implementation of containment measures. CONCLUSIONS: Comparisons between countries are only possible when simultaneously taking into account that the virus did not arrive in all countries simultaneously, absolute numbers are incomparable due to different population sizes, rates per 100,000 of the population are incomparable because not all countries are affected homogeneously, susceptibility to death by COVID-19 can differ between populations, and a death will only be reported as a COVID-19 related death if the patient was diagnosed with SARS-CoV-2 infection. With our methods, we accounted for all these factors and established an unbiased direct comparison between countries. This comparison confirms that early adoption of containment strategies is key in flattening the curve of the epidemic.