Autumn COVID-19 surge dates in Europe correlated to latitudes, not to temperature-humidity, pointing to vitamin D as contributing factor
Stephan Walrand
Scientific Reports, doi:10.1038/s41598-021-81419-w
To determine the factor triggering the sudden surge of daily new COVID-19 cases arising in most European countries during the autumn of 2020. The dates of the surge were determined using a fitting of the two last months of reported daily new cases in 18 European countries with latitude ranging from 39° to 62°. The study proves no correlation between the country surge date and the 2 weeks preceding temperature or humidity but shows an impressive linear correlation with latitude. The country surge date corresponds to the time when its sun UV daily dose drops below ≈ 34% of that of 0° latitude.
Introducing reported seasonal blood 25-hydroxyvitamin D (25(OH)D) concentration variation into the reported link between acute respiratory tract infection risk and 25(OH )D concentration quantitatively explains the surge dynamics. Several studies have already substantiated a 25(OH)D concentration impact on COVID-19 severity. However, by comparing different patient populations, discriminating whether a low 25(OH)D concentration is a real factor underlying COVID-19 severity or only a marker of another weakness that is the primary severity factor can be challenging. The date of the surge is an intrapopulation observation and has the benefit of being triggered only by a parameter globally affecting the population, i.e. decreases in the sun UV daily dose. The results indicate that a low 25(OH) D concentration is a contributing factor to COVID-19 severity, which, combined with previous studies, provides a convincing set of evidence. Most European countries underwent an unexpected surge of daily new COVID-19 cases in autumn (Fig. 1 ), imposing new confinement rules and emergency lockdowns. A commonly reported explanation is the decreasing temperature. The aim of this study is to challenge this assumption against a pure latitude impact.
Author contributions S.W. is the single contributor.
Competing interests The author declares no competing interests.
Additional information
Supplementary Information The online version contains supplementary material available at https ://doi. org/10.1038/s4159 8-021-81419 -w. Correspondence and requests for materials should be addressed to S.W. Reprints and permissions information is available at www.nature.com/reprints. Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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