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Assuming that the COVID-19 risk follows Fig. As a result, more than 15% of the European active population could suffer from a 25(OH)D concentration decrease larger than 50% from August to October. This standard deviation could still be higher in the country population with the presence of older and chronically ill subjects. Two longitudinal studies 6, 7 followed one single cohort over 12 months, with one 7 reporting the 25(OH)D concentration curves for each subject allowing a rough estimation of the intraindividual standard deviation, giving a drop of 26 ± 25% in normal subjects (age 31 ± 3 years). Studies in Europe 4, 5, 6, 7, 8 reported a seasonal 25(OH)D concentration drop of 20–26% between August and October (Table 1). Where D(t) is the 25(OH)D concentration expressed in nmol/L units at time t.įigure 2D shows the corresponding daily new case (Eq. To evaluate the impact of UV insolation on the new daily case dynamics, we consider the simple model: Note that as the exponential coefficient varies with time, the doubling time around the surge date is not simply ln(2) divided by this coefficient. This further allows us to prevent overfitting of the data noise by a steepness tuned for each country. \(\gamma\) was assumed to be country independent, as we searched for an impact of latitude on its own. ( 1) visually becomes different from the monoexponential (see supplementary file). \(\) this choice corresponds to the date when Eq. The date of the surge was automatically determined by fitting the two last months of the daily new COVID-19 cases with the empirical model: Īll these data are in the supplementary Excel file. The reinforcement dates of the safety measure were obtained from the Public Health and Social Measures (PHSM) index of the WHO at. The theoretical sun UVB daily dose for vitamin D production, as a function of latitude and of the day of the year, was derived from the digitalization of Fig. School opening dates of 15 out of the 18 countries studied were found at. The average temperature and humidity were computed between 8h00 to 20h00, outside this period, the population is mostly indoors. For each country, an airport close to the PWC was chosen (see supplementary Excel file). The averaged 2-week temperatures and humidity preceding the surge dates were computed from, which collects the archives of all airport weather stations in the world. The country population weighted centre (PWC) latitudes were obtained from the Baylor University population resource ( ). The countries’ daily new COVID-19 cases, more exactly, the daily new SARS-CoV-2 seropositive cases, were obtained from the European Union agency European Centre for Disease Prevention and Control ( ). 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. 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. 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. Several studies have already substantiated a 25(OH)D concentration impact on COVID-19 severity. 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. The country surge date corresponds to the time when its sun UV daily dose drops below ≈ 34% of that of 0° latitude. 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 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°. To determine the factor triggering the sudden surge of daily new COVID-19 cases arising in most European countries during the autumn of 2020.