Abstract: www.nature.com/scientificreports OPEN Autumn COVID‑19 surge dates in Europe correlated to latitudes, not to temperature‑humidity, pointing to vitamin D as contributing factor Stephan Walrand 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. Materials and methods Data sources. 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 (https​://www.ecdc.europ​a.eu/en/publi​catio​ns-data/downl​oad-today​s-data-geogr​aphic​-distr​ibuti​oncovid​-19-cases​-world​wide). The country population weighted centre (PWC) latitudes were obtained from the Baylor University population resource (http://cs.ecs.baylo​r.edu/~hamer​ly/softw​are/europ​e_popul​ation​_weigh​ted_cente​rs.txt). The averaged 2-week temperatures and humidity preceding the surge dates were computed from https​://rp5. ru, which collects the archives of all airport weather stations in the world. For each country, an airport close to the PWC was chosen (see supplementary Excel file). The average temperature and humidity were computed between 8h00 to 20h00, outside this period, the population is mostly indoors. School opening dates of 15 out of the 18 countries studied were found at https​://eacea​.ec.europ​a.eu/natio​ nal-polic​ies/euryd​ice/sites​/euryd​ice/files​/schoo​l_calen​dar_2020_21_0.pdf. 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. 1B from ­reference1. Cliniques Universitaires Saint-Luc, 1200 Brussels, Belgium. email: stephan.walrand@uclouvain.be Scientific..