The autumn COVID-19 surge dates in Europe are linked to latitudes and not to temperature, nor to humidity, pointing vitamin D as a contributing factor

Purpose Determining the triggering factor of the sudden surge of the daily new COVID-19 cases arising in most European countries during 2020 Autumn. Methods The dates of the surge were determined using a fitting of the two last months reported daily new cases in 18 European countries of latitude ranging from 39° to 62°. Results The study proves no correlation between the country surge date and its 2 weeks preceding temperature or humidity, but shows an impressive linear correlation with its 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 reported link between acute respiratory track infection risk with 25(OH)D concentration quantitatively explains the surge dynamics. Conclusions Several studies already substantiated a 25(OH)D concentration impact on COVID-19 severity. However by comparing different patients populations, discriminate whether low 25(OH)D concentration is a real factor of covid-19 severity or only a marker of another weakness being the primary severity factor can be challenging. The date of the surge is an intrapopulation observation and has the benefit to be only triggered by a parameter globally affecting the population, i.e. the sun UV daily dose decreases. The results support that low 25(OH)D concentration is thus well a contributing factor of COVID-19 severity, which joined with the previous studies makes a convincing bundle of evidence