Introduction
A COVID-19 epidemic started in China and then disseminated to other Asian countries before becoming a pandemic. There is a large variability across countries in both incidence and mortality, and most of the current debates on COVID-19 focus on the differences between countries. German fatalities are strikingly low as compared to many European countries. Among the several explanations proposed, an early and large testing of the population was put forward 1 as well as social distancing. However, little attention has been given to regional within-country differences that may propose new hypotheses.
It would appear that the pandemic has so far resulted in proportionately fewer deaths in some Central European countries, the Balkans, China, in most Eastern Asian countries as well as in many Sub-Saharan African countries. Several reasons can explain this picture. One of them may be the type of diet in these low mortality countries. 2
Diet has been proposed to mitigate COVID-19. 3 Some foods or supplements may have a benefit on the immune response to respiratory viruses. However, to date, there are no specific data available to confirm the putative benefits of diet supplementation, probiotics, and nutraceuticals in the current COVID-19 pandemic.4 News and social media platforms have implicated dietary supplements in the treatment and prevention of COVID-19 without evidence. 5
In this paper, we discuss country and regional differences in COVID-19 deaths. We attempt to find potential links between foods and differences at the national or regional levels in the aim to propose a common mechanism focusing on oxidative stress that may be relevant in COVID-19 mitigation strategies. We used cabbage and fermented vegetable as a proof-of-concept.
Biases to be considered
According to the Johns Hopkins coronavirus resource center (https://coronavirus.jhu.edu ), one of the most important ways of measuring the burden of COVID-19 is mortality. However, death rates are assessed differently between countries and there are many biases that are almost impossible to assess. Using the rates of COVID-19 confirmed cases is subject to limitations that are similar to or even worse than the differences in the use of COVID-19 testing.
Differences in the mortality rates depend on health care systems, the reporting method and many unknown factors. Countries throughout the world have reported very different case fatality ratios - the number of deaths divided by the number of confirmed cases - but these numbers cannot be compared easily due to biases. On the other hand, for many countries, the methodology used to report death rates in the different regions is standardized across the country.
We used mortality per number of inhabitants to assess death rates, as proposed by the European Center for Disease Prevention and Control (ecdc, https://www.ecdc.europa.eu/en), and to report trends with cutoffs at 25, 50, 100 and 250 per million.
Our hypothesis is mostly based on ecological data that are hypothesis-generating and that require confirmation by proper studies.